feat(synthesis): cache per-article map extracts to skip redundant LLM calls

[bryanmatthewsimonson]

Problem

Corpus synthesis re-ran the entire map (one LLM call per member) on every "Analyze corpus" click. Nothing persisted the per-article extracts — xray-audits stored only runs/predictions/resolutions/case-briefs, and only the final brief was saved. So running synthesis 3× over a 129-member case paid for ~3× the map, which is the bulk of the cost (129 map calls vs 1 reduce). (The audit runs cache, keyed by article hash, is a separate feature; the corpus map path was uncached.)

Fix

A corpus-extracts store (xray-audits v3), keyed by corpusExtractKey — a SHA-256 over the exact map inputs: sent text + claim digest + case framing + CORPUS_PROMPT_VERSION.

• The runner checks the cache before the confirm dialog, so the spend preview reads "N of M cached — reused for free."
• Cached members short-circuit with no LLM call; misses call and persist the extract.
• The reduce still runs each time (one call, must see the full current extract set).

Effect: after run 1, a re-run over an unchanged corpus costs only the reduce call; adding one article costs one map call, not 129.

Invalidation falls out of the key

• Body edit → memberText changes → new key.
• A Suggest pass adds claims → claim digest changes → that article (and only it) re-maps — which is exactly right (deliberately keyed on inputs, not the article hash alone, or it would serve a stale extract after claims were added).
• Prompt bump → version changes → new key.

Quality

No wire kind, no prompt change — identical outputs, brief quality unchanged. This is pure cost reuse. (Pairs with #191: the claim-join fix enriches inputs; this makes re-running cheap enough to A/B the brief.)

Verification

• New tests: corpus-extracts store CRUD + v1/v2 coexistence across the v3 upgrade; corpusExtractKey determinism + each invalidation trigger. Updated the backup dump-shape assertion for the new store.
• npm test → 1973 pass · npm run build clean · npm run lint 0 errors.

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