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.

components/setup-dialog.tsx

@@ -49,7 +49,7 @@ import type { ThinkingLevel } from "@earendil-works/pi-agent-core";
 /** The pickers' slot identity — same roster language as the Agent
  *  section's slot rows. */
 const SLOT_META: Record<ModelSlotName, { label: string; roster: string }> = {
-	strong: { label: "Strong", roster: "Prober + Teacher" },
+	strong: { label: "Strong", roster: "Prober, teacher, spine, practice trio" },
 	cheap: { label: "Cheap", roster: "Researcher, demo builder, extractor, auditor" },
 	vision: { label: "Vision", roster: "Visual maker + PDF reader" },
 };