digest-aware serialized extraction + chain simplification replace the spine stage

Owner live-test verdict on the spine (2026-09-30): no perceptible gain —
the real problems (restatement nodes, shallow re-derivations, same
conclusion from many angles) are mint-time issues a post-hoc chain
minter cannot fix. This arc moves assembly INTO the extractor and adds
the deterministic dedupe the owner specified:

- Digest-aware extraction: each stretch's extractor prompt carries THE
  GRAPH SO FAR (its own source's merged propositions + chains, ids
  verbatim); build on existing ids instead of restating, lean on
  established conclusions as premises. Mixed-namespace chain endpoints
  (fresh p-ids OR existing n_-ids) validated at the submit seam.
- Serialized per source: a source's batches/chunks run strictly in
  order (each extract reads every earlier merge); cross-source runs
  stay parallel; source-wide abort stops the loop. The spine stage,
  role, phase, usage stage, cost row, and setup roster are gone.
- Core-ideas prompt: goal reframed as uncovering core ideas and showing
  how everything derives from the foundations; direction upward, prefer
  lean derivations, never re-derive what the graph already holds.
- Chain simplification (merger maintenance): simplifyChains drops
  provably redundant chains at every merge — prefer-fewest-premises +
  ancestry-cover ([A,B]⟹X with [X,C]⟹Y covers [A,B,C]⟹Y, transitive);
  same-key provenance lines drop together; rewriteLedgers under the
  audit lock; droppedChains counted separately in stats.

Chunk budget stays 8192 (owner: keep for now, maybe lower later).

src/core/settings.ts

@@ -45,7 +45,6 @@ export const BACKGROUND_MODEL_ROLES = [
 	"demoBuilder",
 	"visualMaker",
 	"extractor",
-	"spine",
 	"auditor",
 	"partitioner",
 	"problemGenerator",
@@ -227,10 +226,6 @@ export const ROLE_SLOT: Readonly<Record<ModelRole, ModelSlotName>> = {
 	researcher: "cheap",
 	demoBuilder: "cheap",
 	extractor: "cheap",
-	// The spine (2026-09-30): cross-stretch assembly — judgment-dense
-	// work over the whole same-source digest, strong tier like the
-	// practice trio.
-	spine: "strong",
 	auditor: "cheap",
 	visualMaker: "vision",
 	pdfReader: "vision",