refactor(retrieval): freeze the v1.5 retrieval, ingestion and citation seams

[mrsibe]

What does this PR do?

Introduces the retrieval, ingestion and citation contracts that v1.5 builds on, with no user-visible behavior change.

Why?

The three v1.5 workstreams all need the same seams, and none existed:

Trust      #155 → #156 → #157      (#94 → #157)
Source     #95 → #98 → #158
Retrieval  #95 → #78 → #96 → #77

Without this step, each issue would grow its own version of retrieval, ingestion and citation, and KnowledgeService would absorb all of them. This is step 0 of epic #154.

Related issue

Fixes #160
Related to #154

What changed

• Retrieval contract — Retriever.search(request: RetrievalRequest): Promise<RetrievalResult>; RetrievalResult.trace records strategy, scope, topK, threshold, durationMs via a pure buildRetrievalTrace().
• KnowledgeService.retrieve() is the new entry point and keeps the embedding-space precondition. The legacy search() keeps its SearchResult[] shape and now delegates to retrieve(), so existing callers are unchanged.
• A requested source filter is refused, not ignored (assertFilterSupported) until [Feat] Source scope: retrieve from the current or selected sources, persisted per session #94 implements pre-KNN filtering. The dead VectorStore.QueryOptions.filter — declared but never read, with no matching clause in knnQuerySql() — is removed. Declaring a capability that does nothing is what this issue exists to stop.
• CitationCandidate declared in shared/types/citation.ts for [Feat] Exact-passage citation: the model cites source spans, not chunks #155: the cited span exists before the answer, so no post-hoc semantic alignment is needed.
• Ingestion run/state contract in src/main/services/ingestion/types.ts: IngestionStage, IngestionRun, the SourceState / IndexState split, and isTerminalStage(), for [Feat] Ingestion runs: persistent pipeline state, retry, and non-destructive re-index #95.

How was this tested?

• npm run typecheck — passes (node, web, test).
• npm test — 345 pass, 0 fail (5 new: test/retrievalContract.test.ts, test/ingestionTypes.test.ts).
• npm run build — passes.
• Eval baseline acceptance: ran the RAG harness against a temp output dir —
  electron . --eval-harness --eval-out=/tmp/eval-out — and the deterministic report is byte-for-byte identical to docs/eval/baseline-v1.4.json (diff clean). docs/eval/ in the repo was not modified.
• Not run: npm run smoke:packaged (no change to Electron/packaging/db-init; the only new module is a pure types entry under src/main/services/ingestion/).

Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes.
• I have tested the affected user workflow. (Retrieval path unchanged; eval metrics identical.)
• I have not included unrelated changes.
• I have updated documentation when necessary. (None required; contracts carry their rationale inline.)

Desktop / build changes

• Not applicable