Extraction observability: usage capture, live status payload, true abort (Wave 4)

Extraction is real spend and background work; the dock needs to see
and stop it. User decisions: persist all cost data (chat record),
per-stage usage granularity, and TRUE cancellation for abort.

USAGE CAPTURE (live + persisted):
- extraction.ts: per-call onStepFinish captures each billed step's
  usage (SDK LanguageModelUsage -> CallUsage: input/output/cache
  read+write/reasoning) into the registry's live per-stage usage
  (extractor/auditor; the merger is deterministic — no model call).
- /api/graph/status runs now carry usage (model + steps per stage);
  cost/context folds are the consumer's (core/prices.ts — nothing
  stores dollars).
- ChatRecord.extractions: append-only per-run usage log (mirrors the
  turns log): startedAt key, at, durationMs, merge stats when the
  merger ran, stages. Persisted on success AND failure — a failed run
  still spent money. Idempotent per startedAt; the list payload
  (ChatSummary) never leaks it.
- EmissionStats moved to contracts/graph.ts (its contract home);
  core/graph.ts re-exports for existing importers.
- Verified end-to-end against the real pipeline (backfill one chat):
  per-step token counts, reasoning tokens, and the auditor's growing
  prompt-cache reads captured and persisted.

TRUE ABORT (the dock's abort button, user decision):
- RunningExtraction owns one AbortController per run, shared across
  phases; abortExtraction(chatId) fires it and returns whether a run
  was aborted (idempotent no-op otherwise). The pending model call
  receives the signal — the provider cancels the request, spend
  stops. throwIfAborted() at phase boundaries ends runs aborted in
  sync gaps.
- Abort is a deliberate stop, not a failure: same ledger discipline
  (nothing merged, nothing checkpointed, partial usage persisted) but
  the run reports zero emissions instead of throwing. Exported via
  src/server/observability.ts for master's /api/activity/abort route.
- Tests: mid-run abort (hanging mock that rejects on the signal),
  live-accumulation + persisted-record assertions, store-seam
  idempotency, and the summary-payload leak guard.

src/core/contracts/chat.ts

@@ -9,6 +9,7 @@
 import type { UIMessage } from "ai";
 import type { Phase3 } from "../phases.ts";
 import type { PlanNodeStatus, PlanTree } from "./plan.ts";
+import type { EmissionStats } from "./graph.ts";
 
 // ─── Chat records (the session store's on-disk format) ───────────────────────
 
@@ -32,6 +33,41 @@ export interface ChatRecord {
 	 * each entry onto its message; aggregates are folds over this log
 	 * (core/prices.ts). */
 	turns?: TurnUsage[];
+	/** Append-only background-extraction usage log (2026-09 observability):
+	 *  one entry per finished pipeline run on THIS chat — extraction is real
+	 *  spend too. Same parallel-log discipline as `turns` (nothing rides
+	 *  inside messages). Keyed by startedAt; idempotent appends. */
+	extractions?: ExtractionRunUsage[];
+}
+
+// ─── Background-extraction usage (the money channel, extraction side) ────────
+
+/** One pipeline stage's model usage — the stage's model + every step it
+ *  billed (CallUsage per agent step). Steps, not a sum: cost and context
+ *  folds happen at read time (core/prices.ts). */
+export interface ExtractionStageUsage {
+	/** The OpenRouter model id that ran this stage. */
+	model: string;
+	steps: CallUsage[];
+}
+
+/** One finished (or failed mid-flight) extraction run's usage record.
+ *  Appended to the chat's `extractions` log when the run ends — success or
+ *  failure, because money was spent either way; `stats` is present only
+ *  when the merger actually ran. */
+export interface ExtractionRunUsage {
+	/** The run's registry startedAt (ms) — the idempotency key. */
+	startedAt: number;
+	at: string;
+	durationMs: number;
+	/** What the run merged into the graph; absent when it failed first. */
+	stats?: EmissionStats;
+	/** Model usage per stage, in pipeline order (merger is deterministic —
+	 *  no model call, no entry). */
+	stages: {
+		extractor: ExtractionStageUsage;
+		auditor: ExtractionStageUsage;
+	};
 }
 
 // ─── Usage accounting (the money channel) ─────────────────────────────────────