feat(chat): consume CR-5 history windowing — server-windowed cold loads + show-earlier backfill

Re-pinned [email protected]>0.10.0 + [email protected]>0.6.1 (reply
frontend-history-windowing-handoff.md); re-mirrored both .dispatch references.

- HistorySync port gains optional { limit?, beforeSeq? } (CR-5 params); the
  app's createHistorySync appends them to GET /conversations/:id.
- COLD-cache fresh load now fetches ?sinceSeq=0&limit=<floor(0.75xL)> — a huge
  conversation no longer ships whole to show 192 chunks. A warm-cache tail sync
  stays unwindowed (windowing a tail that outgrew the limit would leave a
  silent seq gap behind the cache).
- hasEarlier now derives from the [email protected] CONTRACT (1-based gap-free seqs):
  loaded window starting above seq 1 => older history exists — covering both
  locally-trimmed AND server-windowed transcripts (the watermark stays as the
  merge floor only).
- showEarlier(): local cache first; when the cache doesn't reach far enough
  back, backfills the missing older run via ?beforeSeq=<oldestKnown>&limit=
  and persists it (next page-in is local). latestSeq windowed-read caveat is
  satisfied structurally (tail cursor derives from the cache's max seq).
- live-probe: +6 CR-5 checks (seq origin, newest-k ascending, short-chat
  exactness, beforeSeq paging, 400 validation x2). NOT yet run live — backend
  was down at commit time; run pending.
- backend-handoff.md: CR-5 RESOLVED, pins/mirrors current. 602 tests green x2.

1 files changed, 32 insertions, 20 deletions

diff --git a/src/core/chunks/trim.ts b/src/core/chunks/trim.ts
index 1733027..94065b3 100644
--- a/src/core/chunks/trim.ts
+++ b/src/core/chunks/trim.ts
@@ -110,40 +110,52 @@ export function windowTranscript(state: TranscriptState, maxCommitted: number):
 }
 
 /**
- * Page earlier (unloaded) history back in — the "Show earlier messages" action.
+ * The oldest LOADED seq — the start of the transcript's loaded window. Usually
+ * `committed[0].seq`; falls back to the watermark when a trim emptied the
+ * committed list (all-provisional overflow). 0 = window start unknown/origin.
+ */
+function oldestLoadedSeq(state: TranscriptState): number {
+	return state.committed[0]?.seq ?? state.hiddenBeforeSeq;
+}
+
+/**
+ * Page earlier history back in — the "Show earlier messages" action.
  *
- * `earlier` must be ALL locally-known chunks below the watermark (typically the
- * full cached conversation; chunks at/above the watermark are ignored). The
- * newest `count` of them are merged back in front of `committed` and the
- * watermark lowers to the new oldest loaded seq — or clears to 0 when this
- * restore exhausts the known earlier history (nothing left to offer).
+ * `earlier` is every locally-known chunk older than the loaded window
+ * (typically the full cached conversation, possibly extended by a CR-5
+ * `?beforeSeq=` backfill; chunks at/inside the window are ignored). The newest
+ * `count` of them are merged back in front of `committed`, and the watermark
+ * follows the new window start so history merges still can't resurrect what
+ * remains unloaded. Identity when the window already starts at seq 1 (the
+ * contractual origin) or nothing older is known locally.
  */
 export function restoreEarlier(
 	state: TranscriptState,
 	earlier: readonly StoredChunk[],
 	count: number,
 ): TranscriptState {
-	if (state.hiddenBeforeSeq <= 0) return state;
-	const below = earlier.filter((c) => c.seq < state.hiddenBeforeSeq).sort((a, b) => a.seq - b.seq);
-	if (below.length === 0) {
-		// Nothing is actually hidden below the watermark: clear it so the
-		// "Show earlier" affordance disappears.
-		return { ...state, hiddenBeforeSeq: 0, hiddenThinkingCount: 0 };
-	}
+	const oldest = oldestLoadedSeq(state);
+	if (oldest <= 1) return state;
+	const below = earlier.filter((c) => c.seq < oldest).sort((a, b) => a.seq - b.seq);
+	if (below.length === 0) return state;
 	const keep = below.slice(-Math.max(1, count));
-	const exhausted = keep.length === below.length;
 	const firstKept = keep[0];
 	return {
 		...state,
 		committed: [...keep, ...state.committed],
-		hiddenBeforeSeq: exhausted || firstKept === undefined ? 0 : firstKept.seq,
-		hiddenThinkingCount: exhausted
-			? 0
-			: Math.max(0, state.hiddenThinkingCount - countThinking(keep)),
+		hiddenBeforeSeq: firstKept?.seq ?? state.hiddenBeforeSeq,
+		hiddenThinkingCount: Math.max(0, state.hiddenThinkingCount - countThinking(keep)),
 	};
 }
 
-/** Whether unloaded earlier history exists to offer ("Show earlier messages"). */
+/**
+ * Whether earlier history exists below the loaded window — drives the
+ * "Show earlier messages" affordance. Derived from the [email protected] CONTRACT
+ * that per-conversation seqs are 1-based and gap-free: a loaded window that
+ * starts above seq 1 means older chunks exist (locally cached or server-side),
+ * whether the window came from a local trim or a server-windowed (`?limit=`)
+ * fresh load.
+ */
 export function selectHasEarlier(state: TranscriptState): boolean {
-	return state.hiddenBeforeSeq > 0;
+	return oldestLoadedSeq(state) > 1;
 }