feat(runtime): expand A4.2 orchestrated compatibility - #157

Summary

Implements the A4.2a.0 expand-only compatibility layer for managed orchestrated runtime cutover:

• adds immutable assignment/handle snapshots and integrity incident storage;
• adds lifecycle-operation claim, retry, and receipt columns;
• provides application snapshot contracts and PostgreSQL repositories;
• enforces exact tenant and Task -> Run -> Runtime Version -> Assignment binding;
• keeps all new runtime authority/admission gates disabled and introduces no writer cutover.

Design and risk

• Related L2 design: docs/architecture/modules/managed-runtime-orchestrated-cutover.md
• Main failure modes: cross-execution snapshot binding, non-idempotent replay, unsafe downgrade after new writers activate, migration/model drift
• Rollback strategy: expand-only and backward-compatible while writer markers remain absent; downgrade explicitly refuses once any new table or lifecycle writer marker is populated

Verification

• Fast tests pass locally (652 passed, 7 skipped, 69 deselected; coverage 80.96%)
• Lint passes locally for changed files
• New or changed behavior has automated coverage
• Database migrations are backward/forward tested against real PostgreSQL (11 passed)
• Snapshot repositories are tested against real PostgreSQL (1 passed)
• alembic upgrade head and alembic check pass with no schema drift
• Documentation and roadmap are updated
• No credentials, private prompts, production data, or generated runtime artifacts are included

Reviewer notes

Please focus on migration 0049, downgrade refusal markers, canonical snapshot replay semantics, and the execution-chain binding checks in runtime_repositories.py.