[Code Health][P2] Add opt-in real-store integration contract tests

[beagle1903]

The code-health audit found a verification gap: persistence and graph-store tests use fakes and do not execute real PostgreSQL, Neo4j, or Chroma behavior.

Evidence: tests/test_neo4j_store.py:24-82,222-234, tests/test_snapshot_publication.py:41-66,179-197, tests/test_chroma_store.py:10-50, and tests/test_dashboard.py:58-75,872-899. Current tests cannot prove real Cypher acceptance, metadata filters, PostgreSQL serializer/reconnect behavior, or transaction semantics.

Acceptance criteria:

• Default offline tests remain service-free.
• An explicitly marked opt-in suite uses disposable isolated local stores and deterministic fakes for market/model inputs.
• PostgreSQL proves close/recompile/restore and receipt identity/retry behavior.
• Neo4j proves activation failure preserves the prior pointer and immutable conflicts fail closed.
• Chroma proves version/digest filtering and inactive-stage exclusion.

This is a coverage gap, not a confirmed store defect. See the code-health summary and test-coverage-reviewer.md for the scoped prompt.

[beagle1903]

DESIGN_READY\n\n- Classification and owner: ordinary verification/coverage work within accepted store contracts; implementation_worker (gpt-5.6-sol, medium).\n- Authorization: user requested backlog cleanup on 2026-09-20.\n- Scope: add an explicitly opt-in, disposable local-store integration suite with deterministic market/model doubles. Exercise PostgreSQL checkpoint close/recompile/restore and receipt retry semantics; Neo4j activation-failure pointer preservation and immutable conflict behavior; and Chroma version/digest filtering plus inactive-stage exclusion.\n- Non-goals: no production service, credential, data, provider, finance-policy, graph-state, persistence-contract, migration, or external-financial-write change. Default offline tests remain service-free.\n- Invariants and interfaces: existing application adapters and ADR 0006/0007/0008/0019/0024 contracts remain authoritative. Test-only environment state must be isolated, disposable, and explicitly selected. JSON/state impact: none.\n- Acceptance and verification: opt-in marker is documented; suite safely skips or fails with actionable setup guidance when not explicitly enabled; all stated real-store boundaries run against local disposable services; default suite and standard gates remain green.\n- Documentation and risks: document invocation and isolation in contributor guidance. Escalate for a live-store semantic contradiction, a needed production contract change, nondeterminism, or infrastructure that cannot isolate data.