Replace bare re-export in investigation-storage.js with explicit wrapper
functions that own the canonical identity contract: snapshot.id is the sole
save identity authority. The provider never allocates or changes IDs.
7 new deterministic tests prove: identified snapshots persist under their
own id key, explicit competing id arguments are ignored, A/B remain
independently addressable, unknown IDs return null, and legacy singleton
compatibility is preserved for unmigrated callers.
No application callers modified. UI/routes not migrated.
robbond