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.
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