test(grounding): pin source-order contract for discourse adapter

Sidecar characterization that freezes the deterministic source ordering
of the existing aggregated teaching index, cross-pack chains, and
narrative/example composer outputs.  No dependency on the discourse
planner contract — this is the bridge that protects the next two
phases (ResponseMode classification + structured GroundedFact
accessors) from source-order drift.

5 tests pin: aggregated teaching index key order, cross-pack subject
and object views, narrative composer source ordering, example composer
source ordering.

Authored in worktree 3721; landed here so the main-line sequencing
(characterization -> ResponseMode -> accessors -> planner -> wiring)
can proceed against a stable substrate.

tests/test_grounding_source_characterization.py

@@ -0,0 +1,155 @@
+"""Grounding-source characterization for future discourse planning.
+
+These tests deliberately avoid the discourse planner contract.  They freeze
+the deterministic source ordering that a future GroundingBundle adapter can
+consume without re-learning the current surface composers' hidden order.
+"""
+
+from __future__ import annotations
+
+from chat.cross_pack_grounding import (
+    cross_pack_chains_for_object,
+    cross_pack_chains_for_subject,
+)
+from chat.example_surface import example_grounded_surface
+from chat.narrative_surface import narrative_grounded_surface
+from chat.teaching_grounding import _all_chains_index
+
+
+def test_teaching_chain_index_has_stable_key_order_for_discourse_sources() -> None:
+    """The aggregated teaching view is keyed by stable subject/intent pairs.
+
+    A future discourse adapter may sort these keys before selecting support
+    and relation moves.  Pinning representative keys protects that substrate
+    without depending on any new DiscoursePlan type.
+    """
+    chains = _all_chains_index()
+    observed = [
+        (key, chain.chain_id, chain.corpus_id, chain.connective, chain.object)
+        for key, chain in sorted(chains.items())
+        if key[0] in {"knowledge", "light", "memory", "mother", "parent", "truth"}
+    ]
+
+    assert observed == [
+        (
+            ("knowledge", "cause"),
+            "cause_knowledge_requires_evidence",
+            "cognition_chains_v1",
+            "requires",
+            "evidence",
+        ),
+        (
+            ("light", "cause"),
+            "cause_light_reveals_truth",
+            "cognition_chains_v1",
+            "reveals",
+            "truth",
+        ),
+        (
+            ("light", "verification"),
+            "verification_light_reveals_truth",
+            "cognition_chains_v1",
+            "reveals",
+            "truth",
+        ),
+        (
+            ("memory", "verification"),
+            "verification_memory_requires_recall",
+            "cognition_chains_v1",
+            "requires",
+            "recall",
+        ),
+        (
+            ("mother", "cause"),
+            "cause_mother_precedes_daughter",
+            "relations_chains_v2",
+            "precedes",
+            "daughter",
+        ),
+        (
+            ("parent", "cause"),
+            "cause_parent_precedes_child",
+            "relations_chains_v1",
+            "precedes",
+            "child",
+        ),
+        (
+            ("truth", "cause"),
+            "cause_truth_grounds_knowledge",
+            "cognition_chains_v1",
+            "grounds",
+            "knowledge",
+        ),
+        (
+            ("truth", "verification"),
+            "verification_truth_requires_evidence",
+            "cognition_chains_v1",
+            "requires",
+            "evidence",
+        ),
+    ]
+
+
+def test_cross_pack_subject_helpers_return_canonical_discourse_order() -> None:
+    """Cross-pack subject helpers sort by ``(intent, connective, object)``."""
+    family = cross_pack_chains_for_subject("family")
+    parent = cross_pack_chains_for_subject("parent")
+
+    assert [(c.chain_id, c.intent, c.connective, c.object) for c in family] == [
+        ("cause_family_grounds_identity", "cause", "grounds", "identity"),
+        ("cause_family_supports_memory", "cause", "supports", "memory"),
+    ]
+    assert [(c.chain_id, c.intent, c.connective, c.object) for c in parent] == [
+        ("cause_parent_grounds_understanding", "cause", "grounds", "understanding"),
+    ]
+
+
+def test_cross_pack_object_helpers_return_canonical_discourse_order() -> None:
+    """Cross-pack object helpers sort by ``(intent, subject, connective)``."""
+    parent = cross_pack_chains_for_object("parent")
+    family = cross_pack_chains_for_object("family")
+
+    assert [(c.chain_id, c.intent, c.subject, c.connective) for c in parent] == [
+        (
+            "verification_understanding_requires_parent",
+            "verification",
+            "understanding",
+            "requires",
+        ),
+    ]
+    assert [(c.chain_id, c.intent, c.subject, c.connective) for c in family] == [
+        (
+            "verification_identity_requires_family",
+            "verification",
+            "identity",
+            "requires",
+        ),
+    ]
+
+
+def test_narrative_clause_order_is_stable_for_mixed_sources() -> None:
+    """Narrative surfaces expose the order future relation moves must preserve."""
+    surface = narrative_grounded_surface("family", max_clauses=8)
+    assert surface is not None
+
+    assert surface.index("family grounds identity") < surface.index(
+        "family grounds parent"
+    )
+    assert surface.index("family grounds parent") < surface.index(
+        "family supports memory"
+    )
+    assert "cross_pack_chains_v1 + relations_chains_v1" in surface
+
+
+def test_example_clause_order_is_stable_for_mixed_sources() -> None:
+    """Example surfaces expose reverse-chain order for future example moves."""
+    surface = example_grounded_surface("parent", max_examples=8)
+    assert surface is not None
+
+    assert surface.index("child follows parent") < surface.index(
+        "family grounds parent"
+    )
+    assert surface.index("family grounds parent") < surface.index(
+        "understanding requires parent"
+    )
+    assert "cross_pack_chains_v1 + relations_chains_v1" in surface