fix(memory): make observation removal atomic and invalidate stale indexes

[kevintseng]

Summary

• Run observation removal under one immediate SQLite transaction, including the indexed-text read, FTS update and obsolete-vector deletion.
• Preserve all original state on index failure; keep missing-entity and missing-observation no-op responses.
• Keep archived entities out of FTS when their observations are edited.
• Add seven isolated regression cases and update compiled output and the Unreleased note.

Verification

• Before the fix: the new seven-case suite reported four failures, exposing vector retention and non-atomic deletion.
• Final commit 6efdf61: 78 focused/adjacent tests passed; typecheck passed.
• npm run verify:release passed on the identical source tree before commit, including generated-output parity and 17 entry-point starts.
• A targeted mutation removing vector invalidation made the regression suite fail; source was restored and the worktree is clean.
• Independent full-diff review and replay passed, including archived FTS row assertions and simplification review.

Scope

This fixes synchronous observation-removal consistency. Pending asynchronous embedding freshness is a separate follow-up. No schema migration, production data repair, version bump or release is included. Full-suite/coverage results are left to CI.