test(decisioning): unit tests for ProposalStore + lifecycle helpers + cross-store router (#538-impl)

55 new tests covering the v1.5 primitives landed in the previous commit:

- tests/test_proposal_store.py (25 tests): Protocol conformance,
  state-machine guards, idempotency on commit/mark_consumed, eviction
  (24h draft TTL + 7d post-expiry committed grace), cross-tenant
  safety on get / get_by_media_buy_id, discard idempotency,
  create_dev_proposal_store warns.
- tests/test_proposal_lifecycle.py (24 tests): enforce_proposal_expiry
  (PROPOSAL_NOT_FOUND / PROPOSAL_NOT_COMMITTED / PROPOSAL_EXPIRED with
  correct recovery), validate_capability_overlap (4 axes + None=no-op
  + frozenset()=deny-all + field-path-prefix override),
  validate_overlap_subset_of_wire drift catch, detect_finalize_action.
- tests/test_platform_router_proposal_stores.py (6 tests): cross-store
  consistency hard error, orphan-key check, mixed-version tenants,
  proposal_stores= optionality.
- examples/hello_proposal_manager_v15.py: 161 effective LOC of
  source (well under the design's ≤350 budget). Demonstrates the
  full v1.5 surface: typed Recipe with CapabilityOverlap, finalize-
  capable ProposalManager, ctx.recipes-reading DecisioningPlatform,
  cross-store-checked PlatformRouter wiring.

All 3452 tests passing (55 new + 3397 baseline). ruff + mypy clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bokelley