feat(chat): chat limit — bulk quarter-unload, 75% fresh-load window, show-earlier page-in

Long transcripts no longer grow unbounded: past the chat limit (default 256
chunks, localStorage dispatch.chatLimit) the oldest ceil(limit/4) committed
chunks are unloaded in ONE bulk pass — never one-per-delta (old Dispatch's
scroll-jump-per-step bug) — and only while the reader is stuck to the bottom
(scrolled-up readers defer the trim; it catches up in whole quarters). A fresh
page load windows to the newest floor(0.75*limit). Unloading is purely local
(IndexedDB cache + server keep everything); a hiddenBeforeSeq watermark keeps
history merges from resurrecting unloaded chunks, and a 'Show earlier messages'
affordance pages a quarter back in from the cache with scroll-anchor
preservation. Thinking-collapse render keys stay stable across trims via a
hiddenThinkingCount ordinal base.

- core/chunks/trim.ts: pure policy (trim/window/restore/normalize) + tests
- chat store: chatLimit + canUnload deps, windowed load, showEarlier()
- composition root: dispatch.chatLimit localStorage knob + unload gate wired
  to smart-scroll isAtBottom()
- backend CR-5 OPENED (not a blocker): ?limit=/?beforeSeq= on
  GET /conversations/:id (courier backend-handoff-chat-limit.md)
- scripts/live-probe.ts: fix pre-existing stale TurnMetricsEntry reads
  (m1.usage -> total.usage) that crashed the probe; 17/17 live checks pass

1 files changed, 12 insertions, 8 deletions

diff --git a/scripts/live-probe.ts b/scripts/live-probe.ts
index 2b2880b..7099b44 100644
--- a/scripts/live-probe.ts
+++ b/scripts/live-probe.ts
@@ -204,25 +204,28 @@ async function main() {
 	record("turn 1 committed transcript has assistant text", committedText.length > 0);
 
 	// ─── Metrics: LIVE token + timing ([email protected] usage/step-complete/done) ──────
+	// (TurnMetricsEntry is `{ turnId, steps, total }` — the turn aggregate lives on
+	// `total`, present once the live `done` folded.)
 	const liveTurns = selectOrderedTurnMetrics(t1.metrics);
 	const m1 = liveTurns[0];
+	const m1Total = m1?.total ?? null;
 	record(
 		"turn 1 LIVE metrics: a turn with output tokens",
-		m1 !== undefined && m1.usage.outputTokens > 0,
-		m1
-			? `in=${m1.usage.inputTokens} out=${m1.usage.outputTokens} steps=${m1.steps.length}`
-			: "no turn",
+		m1Total !== null && m1Total.usage.outputTokens > 0,
+		m1Total
+			? `in=${m1Total.usage.inputTokens} out=${m1Total.usage.outputTokens} steps=${m1?.steps.length}`
+			: "no finalized turn total",
 	);
 	if (m1 !== undefined) {
 		const anyGen = m1.steps.some((s) => s.genTotalMs !== undefined);
 		const anyTtft = m1.steps.some((s) => s.ttftMs !== undefined);
 		note(
-			`live timing: durationMs=${m1.durationMs ?? "—"}, ` +
+			`live timing: durationMs=${m1Total?.durationMs ?? "—"}, ` +
 				`genTotalMs present=${anyGen}, ttftMs present=${anyTtft}`,
 		);
 		record(
 			"turn 1 LIVE metrics carries timing (durationMs or step genTotalMs)",
-			m1.durationMs !== undefined || anyGen,
+			m1Total?.durationMs !== undefined || anyGen,
 			"requires the backend runtime to have a clock",
 		);
 	}
@@ -248,10 +251,11 @@ async function main() {
 			applyDurableMetrics(initialMetricsState(), dm.turns),
 		);
 		const d1 = durableMerged[0];
+		const d1Total = d1?.total ?? null;
 		record(
 			"durable /metrics turn has token usage",
-			d1 !== undefined && d1.usage.outputTokens > 0,
-			d1 ? `out=${d1.usage.outputTokens} steps=${d1.steps.length}` : "no turn",
+			d1Total !== null && d1Total.usage.outputTokens > 0,
+			d1Total ? `out=${d1Total.usage.outputTokens} steps=${d1?.steps.length}` : "no turn total",
 		);
 	}