feat(rag,kg): time-decay recency scoring + wire contradiction invalidation (Wave 4)

[urmzd]

Wave 4 — Temporal differentiators

Bounded slice landed; green on go build ./..., go vet ./rag/... ./knowledge/..., and go test for every changed package.

What landed

(a) RAG time-decay / recency scoring

• New types.WithRecency(halfLife time.Duration, weight float64) SearchOption.
• After RRF fusion in rag/internal/pipeline, each hit's fused score is blended with an exponential recency factor exp(-ln2 * age / halfLife):
  score = (1-weight)*score + weight*score*recencyFactor.
• Opt-in: a non-positive half-life is a no-op, so the default ranking is unchanged (backward compatible).
• types.SearchHit gains a Timestamp time.Time field, populated from the document's UpdatedAt (fallback CreatedAt) by both rag/memstore and rag/pgstore during search. A zero timestamp means "unknown age" → no decay (factor 1.0).
• Tests:
  • rag/memstore: TestSearchByEmbeddingPopulatesTimestamp (UpdatedAt preferred, CreatedAt fallback).
  • rag/internal/pipeline: TestPipelineRecencyTieBreak (two equally-relevant docs of different ages indexed in memstore; the recent doc outranks the older one only with WithRecency) and TestPipelineRecencyDisabledByDefault.

(b) KG contradiction invalidation

• Wired the previously-callerless Store.InvalidateRelation into the engine ingestion path.
• After creating a relation for an existing (source, target, type), invalidateSupersededRelations finds active prior relation(s) of the same type and calls InvalidateRelation(old, newValidAt), setting the old edge's InvalidAt.
• Rule-based supersession only (same source+target+type supersedes); no LLM judge. The newly created relation and already-invalidated priors are left untouched.
• Tests in knowledge/internal/engine:
  • TestIngestEpisode_SupersededRelationInvalidated (prior invalidated exactly once at the new relation's ValidAt; new relation not invalidated).
  • TestIngestEpisode_DifferentTypePriorNotInvalidated.
  • TestIngestEpisode_AlreadyInvalidatedPriorSkipped.

Deferred (TODO)

• True transaction-time bitemporal versioning + WithAsOf(validTime, txTime) read path.
• LLM-judged supersession (semantic contradiction detection beyond same source+target+type).
• Recency blending in the KG RRF (SearchFacts) path.

Notes

• gofmt -w . reports pre-existing formatting drift across many unrelated files on the foundation branch; this PR deliberately commits only the 8 files it changed (all gofmt-clean) to keep the diff focused.

Stacked on #38 (foundation) — merge after #38.

[urmzd]

Superseded by #44, which was squash-merged into main (53a6aff) and contains every change from this branch — a dry merge of this branch into main is a no-op. main is green (build/vet/golangci-lint, 39 packages, and 8/8 live validation on gpt-4o-mini). Closing as redundant.