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.

tests/settings-resolution.test.ts

@@ -42,8 +42,8 @@ describe("resolveSettings", () => {
 		assert.equal(eff.appearance.sessionNameSerif, true);
 		assert.equal(eff.appearance.mdRendering.headingWeight, 900);
 		// wave 7 collapse: the master switch ONLY + the redesign's chunk
-		// budget — absent record resolves the factory 4096 (the old config constant)
-		assert.deepEqual(eff.graph, { enabled: true, extractionChunkTokens: 4096 });
+		// budget — absent record resolves the factory 8192 (the 2026-09-30 bump)
+		assert.deepEqual(eff.graph, { enabled: true, extractionChunkTokens: 8192 });
 	});
 
 	it("explicit record values win over every factory default", () => {
@@ -75,8 +75,8 @@ describe("resolveSettings", () => {
 		assert.equal(eff.appearance.proseFontSize, 20);
 		assert.equal(eff.appearance.mdRendering.headingWeight, 600);
 		// wave 7 collapse: the section IS the master switch (absent budget
-		// field = factory 4096 — a pre-redesign record resolves cleanly).
-		assert.deepEqual(eff.graph, { enabled: false, extractionChunkTokens: 4096 });
+		// field = factory 8192 — a pre-redesign record resolves cleanly).
+		assert.deepEqual(eff.graph, { enabled: false, extractionChunkTokens: 8192 });
 		// The chunk budget rides the same resolution: stored value wins.
 		const budgeted = resolveSettings({ version: 1, graph: { extractionChunkTokens: 2000 } });
 		assert.deepEqual(budgeted.graph, { enabled: true, extractionChunkTokens: 2000 });