fix(Alden): preserve OSK metadata removals and verify MLX response identity

[twoimo]

Alden's current PR #27 source has three fail-open or missing-history paths that can misrepresent source changes and localhost model readiness.

Changes:

• Append immutable external_source_removed records for missing room/author metadata in a confirmed complete OSK snapshot; isolate account keys, record metadata_removed, and commit checkpoint/cache updates only after raw records are durable. No source database mutation or downstream deletion claim.
• Reject nested JSON float overflow (1e400) in native Aside response envelopes.
• Require the selected fixed MLX model ID, assistant role, finish_reason=stop, and exact bounded Korean diagnostic response before a local-model verification returns success.
• Add regression tests for account isolation, metadata disappearance and re-addition, cancelled work, nested malformed inputs, incorrect/incomplete/other-model responses, and the downstream source-grounded graph node/edge retraction on removal followed by restoration.
• Record the Mac-session blocker and provenance in a dated implementation note and continuation pointer.

Validation: macOS GitHub Actions run 38066589911, exact validation branch SHA a6c872c2de362f71540e20dbe6219d90839ada04. Python 3.11 focused 40/40 passed (including live-snapshot fixture deletion/readdition and source-grounding), job 114255363711 completed success. Test/source files match the proposed branch; the CI-only workflow is isolated on the validation branch. This is macOS runner synthetic/fake-host testing, not the owner's installed Alden app, MLX model loading, production source/osk-system MCP, physical STT/TTS, or graph view validation.

Scope: 9 files; baseline exactly PR #27 head 0509551b08b71a72778d7e24fa852ccb235e3f7e. This draft stacks on codex/alden-local-runtime-20260927 to avoid an unrelated diff against main. Keep it unmerged until the exclusive Mac copy's uncommitted state and outputs/STATUS.md can be checked, then carry out installed E2E and final release gates.