extraction: the spine stage — cross-stretch assembly between extract and audit

Per-batch pipeline is now extract → spine → audit → merge (owner order,
2026-09-30), the spine inside the serialized audit section so it reads
the graph exactly as the following audit will. Measured motive: 0 of 133
chains in the live corpus crossed a stretch boundary — no stage minted
cross-stretch chains.

- src/agent/subagents/spine.ts: SESSION/NOTES spine preambles + shared
  SPINE_CORE (span doctrine; extraction doctrines carried over), the
  submit_spine TypeBox schema, validateSpine (mechanical span law: every
  chain touches >=1 fresh endpoint AND >=1 existing digest endpoint;
  unknown ids, self-reference, and id collisions are loud), the digest
  prompt builder, and the runner (strong tier, retry-only).
- extraction.ts: 'assembling' phase, spine stage in the per-need gate,
  usage/billing stage, assemble() inside withAuditLock for both front
  doors; first stretch of a source skips the call (empty digest, no
  spend); the audit sees COMBINED propositions (chunk + connectives),
  never chains; merger receives the combined findings.
- core/graph.ts mergeFindings: chain endpoints resolve through the
  job-local map OR pass through verbatim as existing n_ ids (the spine's
  mixed namespace).
- EXTRACTION_CORE: steps-of-reasoning selection bar (examples, notation
  glosses, and restatements that no argument needs are not steps).
- Factory chunk budget 4096 -> 8192 (fewer seams, more local context).
- healStringifiedArrays moved to the shared submit seam (submit-agent.ts).
- Cost panel: Assembly row; usage records carry a spine stage (optional
  on legacy records).
- Tests +5: prompt pins, span-law matrix, digest rendering, seeded
  end-to-end flow (spanning chain + connective, verbatim n_ endpoint,
  spine billed as its own stage), non-spanning refused + retried.

src/core/settings.ts

@@ -45,6 +45,7 @@ export const BACKGROUND_MODEL_ROLES = [
 	"demoBuilder",
 	"visualMaker",
 	"extractor",
+	"spine",
 	"auditor",
 	"partitioner",
 	"problemGenerator",
@@ -226,6 +227,10 @@ 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",
@@ -384,7 +389,7 @@ export const FACTORY_SETTINGS: FactorySettings = {
 		sessionNameSerif: true,
 		mdRendering: { headingWeight: 900, boldColor: null, italicColor: null },
 	},
-	graph: { enabled: true, extractionChunkTokens: 4096 },
+	graph: { enabled: true, extractionChunkTokens: 8192 },
 };
 
 /** The chunk budget's validated floor (GraphSettings.extractionChunkTokens