[0.5.0][RLAC P3-followup] rlacChunkSchemaVersion marker + doc-level post-filter fallback

[erichare]

Follow-up to RLAC P3 (#293). P3 (PR #321) re-tags existing chunks synchronously on the RLAC flip-on transition, which closes the empty-results hole for the normal path. Two gaps remain, both requiring a new persisted workspace field (rlacChunkSchemaVersion) across all four control-plane backends (memory/file/sqlite/astra) — a schema change deferred from P3:

1. Upgrade path. A workspace that already had rlacEnabled: true before 0.5.0 won't re-tag (the transition already happened). Interim documented in docs/rlac.md: toggle rlacEnabled off then on once. A version marker + a one-time startup/opt-in backfill (à la reconcileOrphansOnStart) would remove the manual step.
2. Very large workspaces. The synchronous re-tag runs inside the flip PATCH request; for a workspace with millions of chunks this could be slow. A rlacChunkSchemaVersion marker would let the search path fall back to control-plane doc-level post-filtering for not-yet-backfilled workspaces, so the flip returns immediately and the backfill runs out-of-band.

Scope

• Add rlacChunkSchemaVersion to WorkspaceRecord + all 4 backends (types, converters, astra additive column migration).
• Set it after a successful chunk re-tag (flip-on + the standalone backfill script).
• Gate the search path: when stale, don't push visible_to down; post-filter hits by joining documentId → the control-plane doc's visibleTo (the same compiled policy, evaluated against the doc).
• Extend scripts/backfill-rlac.ts to build drivers and re-tag chunks (today it's file-backend, doc-level only).