proof_chain phase 2.1: the acyclicity guard at the shared binding-graph
construction boundary, before phase 2.2 wiring can build a cyclic-capable structure.
- generate/binding_graph/acyclicity.py: pure find_cycle(adjacency) detector
(deterministic three-colour DFS; isolated, no model import).
- model.py __post_init__: builds {lhs: deps} adjacency over equations and raises
BindingGraphError(circular_dependency ...) on a cycle. Runs on every binding
graph (math + future proof) — illegal states unrepresentable for all consumers.
- tests/test_binding_graph_acyclicity.py: 17 tests (pure checker + construction
enforcement); mutation-verified non-vacuous.
- ADR-0203: new ADDITIVE invariant referencing ADR-0132 (not an amendment —
preserves the why-added-later history).
Math-lane regression proof: the only producer (math adapter) is acyclic by
construction (fresh result symbol per op, deps point backward); full
binding-graph + admissibility surface 392 green; guard refuses no existing graph.
Honesty boundary (load-bearing): through phase 2.3, proof_chain is SOUND OVER
DECLARED ATOMS, not grounded in recognized input (grounding is phase 2.4).
full binding-graph/admissibility surface: 392 passed. smoke: 67 passed.
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