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, 0 deletions

diff --git a/src/features/smart-scroll/ui/controller.svelte.ts b/src/features/smart-scroll/ui/controller.svelte.ts
index 99d53ca..dbe65d1 100644
--- a/src/features/smart-scroll/ui/controller.svelte.ts
+++ b/src/features/smart-scroll/ui/controller.svelte.ts
@@ -22,6 +22,12 @@ export interface SmartScrollController {
 	/** Reactive: show the "scroll to bottom" affordance (the user has scrolled up). */
 	readonly showButton: boolean;
 	/**
+	 * Non-reactive point-in-time query: is the view stuck to the bottom right now?
+	 * For imperative callers (e.g. the chat-limit unload gate) that poll at event
+	 * time rather than subscribing — reads the reducer state, not a rune.
+	 */
+	isAtBottom(): boolean;
+	/**
 	 * Attach to the scroll container; returns a teardown to call on unmount.
 	 * Pass the inner CONTENT element to also follow height changes that aren't a
 	 * transcript update (async markdown/highlight, image loads, a collapse toggling,
@@ -84,6 +90,10 @@ export function createSmartScrollController(): SmartScrollController {
 			return showButton;
 		},
 
+		isAtBottom(): boolean {
+			return state.stuck;
+		},
+
 		attach(node: HTMLElement, content?: HTMLElement): () => void {
 			el = node;
 			node.addEventListener("scroll", handleScroll, { passive: true });