Extraction-UX data layer: the chunk budget becomes a graph setting; the chat view gets a per-session coverage read

Owner-locked extraction-UX redesign (dispatch 2026-09-24), part 1 (data
layer; the UI chunk follows).

THRESHOLD -> SETTING
- config.ts's EXTRACTION.chunkTokens dies; the default moves into
  FACTORY_SETTINGS.graph.extractionChunkTokens (4096, the old constant's
  value). GraphSettings gains the optional field, Required at resolution
  via the standard ?? FACTORY pattern — pre-redesign records resolve the
  default cleanly.
- FLOOR 500 (the implementer's call, documented on the field): every run
  pays the graph-carrying prompt overhead out of the write channel's
  money — a smaller budget fires penny-batches that cost more per
  extracted token than the content is worth. The sanitizer CLAMPS up
  (bd68486 clamp-never-reject discipline: fractions floor, junk drops);
  the settings PUT REJECTS below-floor with the rationale in the message.
  No ceiling: a large budget just delays extraction — the user's stated
  cost/thoroughness tradeoff, not a misconfiguration to police.
- The trigger (turn-service.triggerExtraction) takes the budget as a
  parameter — chat-handler passes effective.graph.extractionChunkTokens,
  the fresh per-turn read, same as every other gate.
- The pipeline's BATCH side threads through the same number:
  ExtractionPipelineDeps gains an OPTIONAL getChunkTokens getter with the
  factory default (Teacher's 24 test fixtures stay untouched; the two
  call-site lines inside extraction.ts are the only cross-band edits,
  flagged). chat-handler + backfill/import-notes wire the live
  effectiveSettings — trigger and batches share one budget, always.

PER-SESSION COVERAGE READ
- sessionCoverage(chatId) (server/chats.ts): the chat view's ring-meter
  block — totalMessages/coveredCount/uncoveredCount/coveredTokens/
  uncoveredTokens (the same coverageOf derivation /api/graph/coverage
  serves, scoped to one chat), running (chat-kind extraction registered
  for this id), and lastFoldedAt (the chat's newest coverage row's fold
  — a cheap single pass; null before any fold). GET /api/chats/[id]
  carries it as coverage: {...}; unknown chat -> null (the route 404s
  first).
- /api/graph/coverage (the all-sessions list) compiles untouched — the
  per-session read is a scoping of it, not a supersession; the UI's
  sidebar/ring chunk decides which surface it consumes.

Tests: factory/sanitizer pins (clamp-up, fraction-floor, junk-drop),
resolution pins (absent -> 4096, stored 2000 wins), settings-body pins
(valid/absent/fraction/below-floor 400/junk 400), sessionCoverage pins
(derived split, newest-fold scoping, cross-session isolation, running via
the registry, null on unknown chat). 893/893.

src/server/chats.ts

@@ -1,6 +1,8 @@
 import { abortExtraction, extractionStatuses } from "../agent/subagents/extraction.ts";
+import { coverageOf } from "../core/coverage.ts";
 import { deleteChatTraces, type ChatDeleteMode, type ChatDeletionStats } from "../core/graph-deletion.ts";
 import { titleFromPrompt } from "../core/contracts.ts";
+import type { UIMessage } from "ai";
 import { graphRevision } from "./graph.ts";
 import { stores } from "./stores.ts";
 import { isTurnRunning } from "./turn-liveness.ts";
@@ -110,3 +112,47 @@ export async function deleteChatSession(chatId: string, mode: ChatDeleteMode): P
 	stores.chatStore.deleteChat(chatId);
 	return { ok: true, mode, stats, jobsRemoved, graphRevision: graphRevision() };
 }
+
+/** One session's coverage read (extraction-UX redesign, owner dispatch
+ *  2026-09-24): the chat view's ring meter + popover data — the same
+ *  derivation the all-sessions list serves (/api/graph/coverage), scoped
+ *  to one chat. "Uncovered" stays DERIVED per request (fingerprint the
+ *  transcript, diff the coverage ledger — never dual-tracked).
+ *
+ *  lastFoldedAt: the newest coverage row's fold timestamp — a cheap single
+ *  pass over the chat's ledger rows (the ring popover's "last extraction"
+ *  line); null before the session's first fold. */
+export interface SessionCoverage {
+	totalMessages: number;
+	coveredCount: number;
+	uncoveredCount: number;
+	coveredTokens: number;
+	uncoveredTokens: number;
+	/** True while a chat-kind extraction run is registered for this session. */
+	running: boolean;
+	/** ISO of the newest fold in this session's coverage rows; null before any. */
+	lastFoldedAt: string | null;
+}
+
+/** The per-session coverage block (see SessionCoverage). Null when the
+ *  chat doesn't exist — callers own the 404. */
+export function sessionCoverage(chatId: string): SessionCoverage | null {
+	const record = stores.chatStore.getChat(chatId);
+	if (!record) return null;
+	const entries = stores.graphStore.readCoverage();
+	const cov = coverageOf(record.messages as UIMessage[], entries, chatId);
+	let lastFoldedAt: string | null = null;
+	for (const entry of entries) {
+		if (entry.chatId !== chatId) continue;
+		if (lastFoldedAt === null || entry.foldedAt > lastFoldedAt) lastFoldedAt = entry.foldedAt;
+	}
+	return {
+		totalMessages: cov.slots.length,
+		coveredCount: cov.coveredCount,
+		uncoveredCount: cov.uncoveredCount,
+		coveredTokens: cov.coveredTokens,
+		uncoveredTokens: cov.uncoveredTokens,
+		running: extractionStatuses().some((run) => run.kind === "chat" && run.chatId === chatId),
+		lastFoldedAt,
+	};
+}