fix(synthesis): member claims join by source_url, not the orbit filter

[bryanmatthewsimonson]

The bug

A real corpus run over a 129-article case surfaced ~2 claims to the LLM. Root cause: buildMemberUnits fed the map/reduce from deriveArticleRows' row.claims, and those rows attach only orbit claims — allClaims.filter(c => c.about.includes(caseEntityId)). Claims are authored about their subjects (SARS-CoV-2, EcoHealth, Fauci…), not the case container, so only the 2 claims someone tagged directly onto the case qualified.

Phase 20.1 made article membership a union (tag ∪ claim-about-case) but left claim-attachment on the old orbit-only definition — so every tag-member article joined with claims: []. Atomized claims were dropped before the model ever saw them.

Confirmed against a live workspace: the case "What is the origin of Covid?" has 2 of 1,882 claims about it; the rest reference 378 subject entities.

The fix (surgical, synthesis-only)

• buildMemberUnits now joins each member's claims by normalized source_url against the full registry (data.claimsById), key-first/oldest-first for determinism. Restores ~1,882 claims across 91 articles.
• The deterministic dossier's own deriveArticleRows attachment is left untouched (governed separately by CASE_DOSSIER_DESIGN.md).
• synthesis-block.js derives the corpus staleness hash from the actual member-claim ids sent, not dossier.orbit.claim_ids, so adding/removing a claim on a member invalidates the stored brief. Stored briefs re-hash once and show "stale" — correct, the corpus definition changed.
• The URL join also sidesteps the claim article_hash freeze (stamped at extraction, lags a re-publish): inclusion keys on URL and re-keys the unit to the member's current hash.

Not in this PR (surfaced, deferred)

• 30 members with no local archive text can't feed the corpus (can't synthesize bytes you don't have). Recapture to include.
• DIGEST_CLAIM_CAP = 150 still bounds the reduce stage's cross-article claim index (the per-article map stage sees each article's full set).

Verification

• New test: buildMemberUnits joins by URL and excludes non-member claims.
• npm test → 1971 pass · npm run build clean · npm run lint 0 errors.

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