feat(platform): persist metadata fabric bindings

[zhouning]

Summary

• add a tenant-scoped, append-only metadata fabric binding ledger with forced row-level security
• bind resource versions to execution plans, policy decisions, approvals, and provider evidence using immutable content hashes
• add PlatformGateway commit/read APIs with separation-of-duties checks and exact-replay idempotency
• add PostgreSQL rehearsal coverage, architecture decision, evidence, roadmap, system-of-record, and CI updates

Verification

• required platform tests: 608 passed
• PostgreSQL integration tests: 4 passed
• focused binding tests: 36 passed
• runtime, migration, and platform truth tests: 29 passed
• frontend production build: passed
• migration catalog: 98 migrations, valid
• PlatformGateway and binding-ledger validators: valid

Stack

This M3-3 PR is based on feat/ar1-metadata-fabric-local-ingestion-replay so its diff contains only the immutable binding-ledger capability. It must not be merged automatically.

[zhouning]

Superseded by replacement PR #92, rebuilt from canonical main and merged as 4c0cdd2. Canonical main CI run 31956028908 completed successfully, including Platform Required Tests, Frontend Build, and Full Agent Evaluation.