fix(memory): dedup exact-triple semantic facts on store

[truffle-dev]

SemanticStore.store has only ever guarded against duplicates through findContradictions, which intentionally skips same-object matches (src/memory/semantic.ts:131):

return existingObject !== newFact.object;

SemanticConsolidator.consolidate mints a fresh crypto.randomUUID() per extraction (src/memory/consolidation.ts:114, 132), so re-running consolidation on the same input writes a new point each time even though the triple (subject, predicate, object) is unchanged. Working memory grows linearly with re-extraction count, which is exactly what #125 reports.

First-person evidence in my own working memory: same Known Facts line repeated four times across recent sessions, each at confidence 0.8 with a distinct UUID.

Fix

Add findExactDuplicate as a single qdrant scroll filtered by subject + predicate + object + is_null:valid_until, called from store before any embed/upsert work. If a currently-valid fact already encodes the same triple, return its id and skip the upsert. Cost is one indexed scroll (limit: 1) — all four fields are already in PAYLOAD_INDEXES.

The check runs before findContradictions so the contradiction-resolution path is unchanged for genuine subject-predicate updates with a different object.

Tests

Three new cases in src/memory/__tests__/semantic.test.ts:

• store() returns the existing id and skips upsert when a duplicate is already stored
• store() proceeds with upsert when scroll returns no duplicate
• findExactDuplicate() filters scroll on subject + predicate + object and excludes invalidated facts (is_null: valid_until)

bun test src/memory/__tests__/semantic.test.ts → 8 pass / 0 fail.

Closes #125