[P0] Retrieval fallback chain (fast→deep→scope-expand→summary)

[justinbuzzni]

Goal

• Reduce no-result user experience by adding deterministic fallback stages.

Scope

• If fast retrieval returns 0 / low-confidence, retry deep.
• If deep is still low-confidence, expand scope once.
• Final fallback returns concise summary candidates.
• Emit reason codes for each fallback stage.

Acceptance Criteria

• No-result cases reduced by >=30% on benchmark set.
• Existing high-confidence queries do not regress in Top1.
• Fallback stage reason logs are available for debugging.

Checklist

• Implement fallback state machine
• Standardize reason codes
• Add unit/integration tests
• Add benchmark comparison report

[justinbuzzni]

Progress update (corrected):

Implemented core fallback chain in Retriever (auto mode):

• primary fast retrieval
• fallback deep retrieval
• fallback scope-expanded deep retrieval
• final summary fallback

Also added fallback trace reason codes in retrieval result (fallbackTrace).

Validation:

• npx vitest run tests/retriever-fallback-chain.test.ts tests/retriever.memu-adoption.test.ts tests/retriever-strategy-scope.test.ts tests/md-mirror.test.ts tests/ingest-interceptor.test.ts ✅ (11/11)
• npm run build ✅

Commit:

• 0dceb2d

[justinbuzzni]

Progress update:

Implemented core fallback chain in Retriever (auto mode):

• primary fast retrieval
• fallback deep retrieval
• fallback scope-expanded deep retrieval
• final summary fallback

Also added fallback trace reason codes in retrieval result (fallbackTrace).

Validation:

• npx vitest run tests/retriever-fallback-chain.test.ts tests/retriever.memu-adoption.test.ts tests/retriever-strategy-scope.test.ts tests/md-mirror.test.ts tests/ingest-interceptor.test.ts ✅ (11/11)
• npm run build ✅

Commit:

• 0dceb2d