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).

tests/settings.test.ts

@@ -23,7 +23,7 @@ describe("FACTORY_SETTINGS (the compiled factory layer)", () => {
 		assert.deepEqual(FACTORY_SETTINGS.general, { autoUpdate: true });
 		// wave 7 collapse: the master switch ONLY + the extraction-UX
 		// redesign's chunk budget (2024-09-24); default doubled 8192 with
-		// the spine stage (owner, 2026-09-30)
+		// the digest-aware redesign (owner, 2026-09-30)
 		assert.deepEqual(FACTORY_SETTINGS.graph, { enabled: true, extractionChunkTokens: 8192 });
 		const a = FACTORY_SETTINGS.appearance;
 		assert.equal(a.mode, "dark");
@@ -203,13 +203,12 @@ describe("sanitizeUserSettings", () => {
 		assert.deepEqual(out.agent?.tools, { quiz: false, demo: true });
 	});
 
-	it("BACKGROUND_MODEL_ROLES matches the nine background roles (spine added 2026-09-30; practice trio 2026-09-29)", () => {
+	it("BACKGROUND_MODEL_ROLES matches the eight background roles (practice trio 2026-09-29)", () => {
 		assert.deepEqual([...BACKGROUND_MODEL_ROLES], [
 			"researcher",
 			"demoBuilder",
 			"visualMaker",
 			"extractor",
-			"spine",
 			"auditor",
 			"partitioner",
 			"problemGenerator",