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/graph.ts

@@ -22,6 +22,17 @@
  *  strength — strength is derived at read time from topology. */
 export type EdgeType = "follows" | "supports";
 
+/** What one extraction run merged into the graph ledgers — the merger's
+ *  output, also persisted in extraction-run usage records (contracts/chat). */
+export interface EmissionStats {
+	/** New graph nodes appended. */
+	nodes: number;
+	/** New edge assertions appended. */
+	edges: number;
+	/** Ledger events rebuilt (quiz outcomes + observations). */
+	events: number;
+}
+
 /** One provenance point — message-granular (DECIDED 2026-09-02): nodes and
  *  edges record every (chat, message) where they came up, deep-linkable to
  *  the exact transcript point. */