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/cost-summary.ts

@@ -61,7 +61,6 @@ export interface GraphCostSummary {
 	 *  historical spend on either is simply not displayed.) */
 	stages: {
 		extraction: StageCost;
-		spine: StageCost;
 		audit: StageCost;
 	};
 	/** Newest first, capped at `recentCap` (default 10). */
@@ -113,16 +112,11 @@ function mergeTokens(into: CallUsage, steps: readonly CallUsage[]): void {
  *  same output — no clock, no fs, no store reads. */
 export function graphCostSummary(input: CostSummaryInput): GraphCostSummary {
 	const extraction = emptyStage();
-	const spine = emptyStage();
 	const audit = emptyStage();
 	const runs: RecentRun[] = [];
 
 	const foldExtractionRun = (run: ExtractionRunUsage, title: string) => {
 		const stages = run.stages ?? { extractor: { model: "unknown", steps: [] }, auditor: { model: "unknown", steps: [] } };
-		// The spine stage is OPTIONAL on the record (2026-09-30): records
-		// written before it existed (and runs whose digest was empty, so
-		// the call was skipped) carry no entry — default to empty steps.
-		const spineStage = stages.spine ?? { model: "unknown", steps: [] };
 		// Per-stage: tokens always count; dollars only when priced.
 		extraction.runs++;
 		mergeTokens(extraction.tokens, stages.extractor.steps);
@@ -130,14 +124,6 @@ export function graphCostSummary(input: CostSummaryInput): GraphCostSummary {
 		if (extDollars === null) extraction.unpricedRuns++;
 		else extraction.dollars += extDollars;
 
-		if (spineStage.steps.length > 0) {
-			spine.runs++;
-			mergeTokens(spine.tokens, spineStage.steps);
-			const spDollars = stageDollars(spineStage.model, spineStage.steps);
-			if (spDollars === null) spine.unpricedRuns++;
-			else spine.dollars += spDollars;
-		}
-
 		audit.runs++;
 		mergeTokens(audit.tokens, stages.auditor.steps);
 		const audDollars = stageDollars(stages.auditor.model, stages.auditor.steps);
@@ -149,7 +135,7 @@ export function graphCostSummary(input: CostSummaryInput): GraphCostSummary {
 		const runDollars = allStages.every((d) => d === null)
 			? null
 			: allStages.reduce<number>((a, d) => a + (d ?? 0), 0);
-		const allSteps = [...stages.extractor.steps, ...spineStage.steps, ...stages.auditor.steps];
+		const allSteps = [...stages.extractor.steps, ...stages.auditor.steps];
 		runs.push({
 			startedAt: run.startedAt,
 			at: run.at,
@@ -177,7 +163,7 @@ export function graphCostSummary(input: CostSummaryInput): GraphCostSummary {
 	return {
 		totalDollars: extraction.dollars + audit.dollars,
 		unpricedRuns,
-		stages: { extraction, spine, audit },
+		stages: { extraction, audit },
 		runs: capped,
 	};
 }