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.
Refines BoundUnknown from "the symbol whose value the solver determines"
to "the symbol at a specific temporal/state index with a specific
question-form". Two new required fields on BoundUnknown — state_index
(initial/terminal/Operation(operation_index)) and question_form
(count/rate/total/difference/ratio/identity) — populated by the new
pure-function resolver in generate/binding_graph/question_target.py.
The adapter (ADR-0133) now delegates Unknown -> BoundUnknown construction
to bound_unknown_from_math_problem_graph. No runtime wiring, no solver
invocation. Phase 5 (bounded-grammar / B3 integration) remains deferred.
Refusal-first via the new QuestionTargetError (sibling of AdapterError /
AdmissibilityError). Closed reason vocab: not_a_math_problem_graph,
unknown_entity_not_in_entities, apply_rate_unit_mismatch,
unmappable_question_form. Closed precedence rule on question_form
documented in ADR-0135 (compare_multiplicative > compare_additive >
apply_rate{numerator|denominator unit-match} > count); ambiguity refuses.
SemanticSymbolicBindingGraph.__post_init__ gains a cross-collection
guard: Operation(operation_index) must satisfy operation_index <
len(equations). canonical_string emission widened to include
state=... form=... tokens (hash differs from Phase 3 main by design —
not a regression; byte-equal across runs preserved).
Parents: ADR-0132 / ADR-0133 / ADR-0134.
Tests: +70 new (45 unit in test_binding_graph_question_target.py +
25 integration in test_binding_graph_adapter_question_target.py); 5
Phase 1+3 BoundUnknown fixtures migrated. Total binding-graph lane
295/1 pass (1 pre-existing test_symbol_binding_uses_slots failure on
Python 3.14, unrelated to Phase 4 — exists on origin/main). Pyright
clean on new and modified files. No edits to algebra/, chat/, core/,
or runtime hot path. Field invariant untouched.
