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, 10 insertions, 1 deletions

diff --git a/src/core/chunks/reducer.ts b/src/core/chunks/reducer.ts
index 7ce55ce..0a57839 100644
--- a/src/core/chunks/reducer.ts
+++ b/src/core/chunks/reducer.ts
@@ -10,6 +10,8 @@ export function initialState(): TranscriptState {
 		currentTurnId: null,
 		latestUsage: null,
 		sealedTurnId: null,
+		hiddenBeforeSeq: 0,
+		hiddenThinkingCount: 0,
 		generating: false,
 	};
 }
@@ -41,6 +43,10 @@ function flushAccumulating(
  * Dedupes by seq (new wins), keeps seq-monotonic order, idempotent.
  * When sealedTurnId is set, drops all provisional chunks (now superseded)
  * and clears sealedTurnId.
+ *
+ * Chunks below the chat-limit unload watermark (`hiddenBeforeSeq`) are
+ * REJECTED: a full-cache or tail merge must not resurrect what the trim
+ * unloaded. Restoring earlier history goes through `restoreEarlier` instead.
  */
 export function applyHistory(
 	state: TranscriptState,
@@ -48,7 +54,10 @@ export function applyHistory(
 ): TranscriptState {
 	const seqMap = new Map<number, StoredChunk>();
 	for (const c of state.committed) seqMap.set(c.seq, c);
-	for (const c of chunks) seqMap.set(c.seq, c);
+	for (const c of chunks) {
+		if (c.seq < state.hiddenBeforeSeq) continue;
+		seqMap.set(c.seq, c);
+	}
 	const committed = Array.from(seqMap.values()).sort((a, b) => a.seq - b.seq);
 
 	if (state.sealedTurnId !== null) {