New module `teaching/math_contemplation_proposal.py` defines the
`MathReaderRefusalShapeProposal` dataclass — the math-domain analog of
`TeachingChainProposal` for the Tier-1 contemplation corridor.
- `build_proposal` enforces all seven invariants: math domain, ShapeCategory
enum membership, ≥2 evidence pointers, valid ChangeKind Literal, JSON-
serializable payload, ≥40-char wrong_zero_assertion, and non-None
reasoning_trace with a non-empty trace_id.
- `canonical_bytes` / `compute_proposal_id` produce stable sha256-based IDs;
evidence reduced to evidence_hash, trace to trace_id for stability.
- `ReasoningTrace` imported under TYPE_CHECKING only (W0/A1 not yet merged);
duck-typed at runtime via trace_id attribute.
- 16 tests cover all eight brief obligations plus freeze and sensitivity checks.
- `core test --suite teaching -q` green (17 passed).
