diff --git a/core-rs/src/vault.rs b/core-rs/src/vault.rs
index f095ee4f..8444b4e3 100644
--- a/core-rs/src/vault.rs
+++ b/core-rs/src/vault.rs
@@ -9,7 +9,9 @@
 //! The CGA inner product in Cl(4,1) is structurally diagonal with
 //! ±1 metric values, so per-versor scoring collapses to
 //!
-//!     sum_i metric[i] * v[i] * q[i]
+//! ```text
+//! sum_i metric[i] * v[i] * q[i]
+//! ```
 //!
 //! which is 32 multiplies + 32 adds, not the 1024-op full
 //! geometric_product the reference scalar path computes.  Bit-
@@ -150,3 +152,180 @@ pub fn vault_reproject_parallel(versors: &[[f32; 32]]) -> Vec<[f32; 32]> {
     use crate::cga::null_project_raw;
     versors.par_iter().map(|v| null_project_raw(v)).collect()
 }
+
+// ===========================================================================
+// Delta-CRDT substrate (ADR-0180 §2.1 / §2.2)
+// ===========================================================================
+//
+// `LocalArena` is the thread-local, share-nothing write cache each modality
+// adapter accumulates into (ADR-0180 §2.1). It never touches global state, so
+// it needs no locks: concurrency safety comes from each thread owning its own
+// arena, not from synchronisation.
+//
+// `SemilatticeDelta` is the join-semilattice contract (ADR-0180 §2.2): the
+// merge of deltas is commutative, associative, and idempotent. The Merge
+// Kernel folds deltas into a single content-addressed, deduplicated, totally
+// ordered set — the order-invariant state the Vault consumes. Ordering is by
+// content (the versor's IEEE-754 bit pattern + provenance bytes), never
+// arrival order, per the §2.2 content-addressed-tiebreak amendment
+// (2026-05-29). That is precisely what makes
+// `hash(Sequential_Ingest) == hash(Concurrent_CRDT_Ingest)` (§4.3) reachable:
+// the merged state cannot depend on the order deltas arrived in.
+//
+// This is the pure-CPU Rust substrate (§1.5.5); no MLX/UMA handshake and no
+// Python binding land here — those are downstream (ADR-0180 §4.1 item 2, and
+// ADR-0181 PR-5 respectively).
+
+use std::cmp::Ordering;
+
+/// One write accumulated into an arena: a Cl(4,1) versor plus opaque
+/// provenance bytes. Provenance is part of the content key, so two writes of
+/// the same versor under different provenance are distinct semilattice
+/// elements (both retained); two byte-identical writes collapse (the
+/// idempotence leg of the join semilattice, ADR-0180 §2.2).
+#[derive(Clone, Debug)]
+pub struct ArenaEntry {
+    pub versor: [f32; 32],
+    pub provenance: Vec<u8>,
+}
+
+impl ArenaEntry {
+    pub fn new(versor: [f32; 32], provenance: Vec<u8>) -> Self {
+        Self { versor, provenance }
+    }
+}
+
+/// Total, arrival-independent content order over arena entries.
+///
+/// f32 has no total `Ord` (NaN is unordered), so we compare the raw IEEE-754
+/// bit patterns — which is exactly what "content-addressed" means here:
+/// byte-identical versors sort together and deduplicate, and `-0.0`/`+0.0`
+/// (distinct bytes) are treated as distinct content, as a byte-addressed merge
+/// requires. Ties on the versor fall through to the provenance bytes, giving a
+/// total order (ADR-0180 §2.2 amendment, mirroring ADR-0181 §2.2's merge key).
+fn content_cmp(a: &ArenaEntry, b: &ArenaEntry) -> Ordering {
+    for i in 0..32 {
+        let o = a.versor[i].to_bits().cmp(&b.versor[i].to_bits());
+        if o != Ordering::Equal {
+            return o;
+        }
+    }
+    a.provenance.cmp(&b.provenance)
+}
+
+#[inline]
+fn content_eq(a: &ArenaEntry, b: &ArenaEntry) -> bool {
+    content_cmp(a, b) == Ordering::Equal
+}
+
+/// A snapshot of newly-ingested entries (ADR-0180 §2.2). A `Delta` is always
+/// held in content-addressed order with byte-identical duplicates removed, so
+/// it is a canonical join-semilattice element regardless of the order its
+/// entries were inserted in.
+#[derive(Clone, Debug, Default)]
+pub struct Delta {
+    entries: Vec<ArenaEntry>,
+}
+
+impl Delta {
+    /// Canonicalise an arbitrary entry list into a Delta: sort by content,
+    /// drop byte-identical duplicates. `sort_by` is stable, but the dedup key
+    /// is the *whole* content, so stability is immaterial to the result.
+    pub fn from_entries(mut entries: Vec<ArenaEntry>) -> Self {
+        entries.sort_by(content_cmp);
+        entries.dedup_by(|a, b| content_eq(a, b));
+        Self { entries }
+    }
+
+    pub fn entries(&self) -> &[ArenaEntry] {
+        &self.entries
+    }
+
+    pub fn len(&self) -> usize {
+        self.entries.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_empty()
+    }
+}
+
+/// The join-semilattice contract for Delta-CRDT state (ADR-0180 §2.2).
+///
+/// Implementors must satisfy, with content-addressed equality (`content_cmp`):
+///   * commutativity   `a.join(b) == b.join(a)`
+///   * associativity   `a.join(b).join(c) == a.join(b.join(c))`
+///   * idempotence     `a.join(a) == a`
+///
+/// These are exercised as failable tests in `tests/test_arena.rs`; if `join`
+/// ever orders by arrival instead of content, or stops deduplicating, those
+/// tests fail loudly (CLAUDE.md §Schema-Defined Proof Obligations).
+pub trait SemilatticeDelta: Sized {
+    fn join(&self, other: &Self) -> Self;
+}
+
+impl SemilatticeDelta for Delta {
+    fn join(&self, other: &Self) -> Self {
+        let mut merged =
+            Vec::with_capacity(self.entries.len() + other.entries.len());
+        merged.extend_from_slice(&self.entries);
+        merged.extend_from_slice(&other.entries);
+        Delta::from_entries(merged)
+    }
+}
+
+/// Thread-local, share-nothing write cache for one modality adapter
+/// (ADR-0180 §2.1). Adapters push entries here lock-free; nothing is ever
+/// written to global state from an arena. `snapshot` emits the order-invariant
+/// `Delta` the Merge Kernel folds.
+#[derive(Clone, Debug, Default)]
+pub struct LocalArena {
+    entries: Vec<ArenaEntry>,
+}
+
+impl LocalArena {
+    pub fn new() -> Self {
+        Self {
+            entries: Vec::new(),
+        }
+    }
+
+    /// Lock-free local write. Push order is irrelevant: `snapshot`
+    /// canonicalises into content-addressed order.
+    pub fn push(&mut self, versor: [f32; 32], provenance: Vec<u8>) {
+        self.entries.push(ArenaEntry::new(versor, provenance));
+    }
+
+    pub fn len(&self) -> usize {
+        self.entries.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_empty()
+    }
+
+    /// Emit a canonical Delta of everything accumulated so far. Does not drain
+    /// the arena — flush/GC policy is the Merge Kernel's concern, not the
+    /// arena's.
+    pub fn snapshot(&self) -> Delta {
+        Delta::from_entries(self.entries.clone())
+    }
+}
+
+/// The Merge Kernel (ADR-0180 §2.2): fold a batch of deltas into a single
+/// content-addressed, deduplicated, totally ordered entry set. The result is
+/// invariant under any permutation of `deltas` (commutativity + associativity)
+/// and under duplicate deltas (idempotence) — the property §4.3's
+/// `hash(Sequential) == hash(Concurrent)` proof rides on.
+///
+/// Implemented as a single canonicalisation of the union rather than a
+/// `fold(join)` chain; `tests/test_arena.rs` pins that the two are
+/// content-equal, so the cheap path can never silently diverge from the
+/// semilattice fold it stands in for.
+pub fn merge_kernel(deltas: &[Delta]) -> Delta {
+    let mut all = Vec::new();
+    for d in deltas {
+        all.extend_from_slice(&d.entries);
+    }
+    Delta::from_entries(all)
+}
diff --git a/core-rs/tests/test_arena.rs b/core-rs/tests/test_arena.rs
new file mode 100644
index 00000000..42e00b22
--- /dev/null
+++ b/core-rs/tests/test_arena.rs
@@ -0,0 +1,175 @@
+//! Delta-CRDT substrate proof obligations (ADR-0180 §2.1 / §2.2).
+//!
+//! Each test is written to FAIL LOUDLY under the specific violation it names
+//! (CLAUDE.md §Schema-Defined Proof Obligations): a schema/trait is only a
+//! proven property when a test would break if the property were silently
+//! removed. The map:
+//!
+//!   * commutativity / associativity / idempotence — the three join-semilattice
+//!     legs ADR-0180 §2.2 claims `Delta::join` satisfies.
+//!   * permutation-invariant `merge_kernel` — the load-bearing property §4.3's
+//!     `hash(Sequential) == hash(Concurrent)` rides on. Fails if `from_entries`
+//!     stops sorting (i.e. orders by arrival).
+//!   * distinct-provenance retention — guards the dedup from collapsing on the
+//!     versor alone, which would silently drop epistemic state (a wrong=0-style
+//!     data-loss hazard at the merge layer).
+
+use core_rs::vault::{merge_kernel, ArenaEntry, Delta, LocalArena, SemilatticeDelta};
+
+/// Deterministic distinct versor per seed.
+fn versor(seed: u8) -> [f32; 32] {
+    let mut v = [0f32; 32];
+    for (i, slot) in v.iter_mut().enumerate() {
+        *slot = (seed as f32) * 0.5 + (i as f32) * 0.125;
+    }
+    v
+}
+
+fn entry(seed: u8, prov: &str) -> ArenaEntry {
+    ArenaEntry::new(versor(seed), prov.as_bytes().to_vec())
+}
+
+/// Canonical, comparable view of a Delta: (versor bits, provenance) per entry,
+/// in the Delta's own order. Two Deltas are content-equal iff these match,
+/// including order — so this also pins that ordering is content-addressed.
+fn keys(d: &Delta) -> Vec<([u32; 32], Vec<u8>)> {
+    d.entries()
+        .iter()
+        .map(|e| {
+            let mut bits = [0u32; 32];
+            for (i, b) in bits.iter_mut().enumerate() {
+                *b = e.versor[i].to_bits();
+            }
+            (bits, e.provenance.clone())
+        })
+        .collect()
+}
+
+fn delta(entries: Vec<ArenaEntry>) -> Delta {
+    Delta::from_entries(entries)
+}
+
+// --- the three join-semilattice legs (ADR-0180 §2.2) ----------------------
+
+#[test]
+fn join_is_commutative() {
+    let a = delta(vec![entry(1, "p1"), entry(3, "p3")]);
+    let b = delta(vec![entry(2, "p2"), entry(3, "p3")]);
+    // Fails if join carries arrival order: a-first vs b-first would differ.
+    assert_eq!(keys(&a.join(&b)), keys(&b.join(&a)));
+}
+
+#[test]
+fn join_is_associative() {
+    let a = delta(vec![entry(1, "p1")]);
+    let b = delta(vec![entry(2, "p2")]);
+    let c = delta(vec![entry(3, "p3")]);
+    assert_eq!(keys(&a.join(&b).join(&c)), keys(&a.join(&b.join(&c))));
+}
+
+#[test]
+fn join_is_idempotent() {
+    let a = delta(vec![entry(1, "p1"), entry(2, "p2")]);
+    // a ∘ a == a — fails if dedup is removed (length would double).
+    let joined = a.join(&a);
+    assert_eq!(keys(&joined), keys(&a));
+    assert_eq!(joined.len(), 2);
+}
+
+// --- the load-bearing property for §4.3 -----------------------------------
+
+#[test]
+fn merge_kernel_is_permutation_invariant() {
+    let d0 = delta(vec![entry(5, "a"), entry(1, "b")]);
+    let d1 = delta(vec![entry(3, "c")]);
+    let d2 = delta(vec![entry(9, "d"), entry(2, "e"), entry(7, "f")]);
+
+    let forward = merge_kernel(&[d0.clone(), d1.clone(), d2.clone()]);
+    let reversed = merge_kernel(&[d2.clone(), d1.clone(), d0.clone()]);
+    let shuffled = merge_kernel(&[d1.clone(), d0.clone(), d2.clone()]);
+
+    // hash(Sequential) == hash(Concurrent): merged state is independent of the
+    // order deltas arrived in. Fails the instant `from_entries` orders by
+    // arrival instead of content.
+    assert_eq!(keys(&forward), keys(&reversed));
+    assert_eq!(keys(&forward), keys(&shuffled));
+}
+
+#[test]
+fn merge_kernel_dedups_duplicate_deltas() {
+    let d = delta(vec![entry(4, "x"), entry(6, "y")]);
+    // Re-ingesting the same delta is a no-op (idempotence at the kernel).
+    let once = merge_kernel(&[d.clone()]);
+    let twice = merge_kernel(&[d.clone(), d.clone()]);
+    assert_eq!(keys(&once), keys(&twice));
+    assert_eq!(twice.len(), 2);
+}
+
+#[test]
+fn merge_kernel_equals_semilattice_fold() {
+    let deltas = [
+        delta(vec![entry(8, "a"), entry(2, "b")]),
+        delta(vec![entry(5, "c")]),
+        delta(vec![entry(2, "b"), entry(9, "d")]), // overlaps the first delta
+    ];
+    let folded = deltas
+        .iter()
+        .fold(Delta::default(), |acc, d| acc.join(d));
+    // The cheap union-then-canonicalise path must equal the explicit
+    // semilattice fold, or the kernel has silently diverged from the trait.
+    assert_eq!(keys(&merge_kernel(&deltas)), keys(&folded));
+}
+
+// --- data-loss / over-dedup guard -----------------------------------------
+
+#[test]
+fn distinct_provenance_is_not_collapsed() {
+    // Same versor, different provenance => two distinct semilattice elements.
+    // Fails if dedup keys on the versor alone (which would drop state).
+    let same_versor = vec![entry(7, "alpha"), entry(7, "beta")];
+    let merged = delta(same_versor);
+    assert_eq!(merged.len(), 2);
+
+    // Byte-identical content (same versor + same provenance) => collapses.
+    let identical = vec![entry(7, "alpha"), entry(7, "alpha")];
+    assert_eq!(delta(identical).len(), 1);
+}
+
+#[test]
+fn merge_result_is_content_sorted() {
+    let d = merge_kernel(&[delta(vec![entry(9, "z"), entry(1, "a"), entry(5, "m")])]);
+    let ks = keys(&d);
+    let mut sorted = ks.clone();
+    sorted.sort();
+    assert_eq!(ks, sorted, "merge output must be in content-addressed order");
+}
+
+// --- LocalArena (ADR-0180 §2.1) -------------------------------------------
+
+#[test]
+fn arena_snapshot_independent_of_push_order() {
+    let mut a = LocalArena::new();
+    a.push(versor(3), b"p3".to_vec());
+    a.push(versor(1), b"p1".to_vec());
+    a.push(versor(2), b"p2".to_vec());
+
+    let mut b = LocalArena::new();
+    b.push(versor(2), b"p2".to_vec());
+    b.push(versor(3), b"p3".to_vec());
+    b.push(versor(1), b"p1".to_vec());
+
+    // Two arenas fed the same writes in different orders snapshot to the same
+    // canonical Delta (ADR-0180 §2.1: push order is irrelevant).
+    assert_eq!(keys(&a.snapshot()), keys(&b.snapshot()));
+}
+
+#[test]
+fn arena_snapshot_does_not_drain() {
+    let mut a = LocalArena::new();
+    a.push(versor(1), b"p1".to_vec());
+    let _ = a.snapshot();
+    // Flush/GC is the Merge Kernel's concern, not the arena's; snapshot must
+    // be non-destructive so a delayed merge (the §3.2 window) cannot lose it.
+    assert_eq!(a.len(), 1);
+    assert_eq!(a.snapshot().len(), 1);
+}