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, 8 insertions, 7 deletions

diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index 379805f..999f2be 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -25,7 +25,7 @@ import {
 	subscribe as protocolSubscribe,
 	unsubscribe as protocolUnsubscribe,
 } from "../core/protocol";
-import type { ChatStore, MetricsSync } from "../features/chat";
+import type { ChatStore, HistorySync, MetricsSync } from "../features/chat";
 import { createChatStore } from "../features/chat";
 import type { ConversationCache } from "../features/conversation-cache";
 import { createConversationCache } from "../features/conversation-cache";
@@ -111,12 +111,13 @@ export interface CreateAppStoreOptions {
 	localStorage?: Storage;
 }
 
-function createHistorySync(
-	httpBase: string,
-	fetchImpl: typeof fetch,
-): (conversationId: string, sinceSeq: number) => Promise<ConversationHistoryResponse> {
-	return async (conversationId: string, sinceSeq: number) => {
-		const url = `${httpBase}/conversations/${encodeURIComponent(conversationId)}?sinceSeq=${sinceSeq}`;
+function createHistorySync(httpBase: string, fetchImpl: typeof fetch): HistorySync {
+	return async (conversationId, sinceSeq, window) => {
+		let url = `${httpBase}/conversations/${encodeURIComponent(conversationId)}?sinceSeq=${sinceSeq}`;
+		// CR-5 windowing ([email protected]): both must be positive
+		// integers when present (the server 400s otherwise; callers guarantee it).
+		if (window?.limit !== undefined) url += `&limit=${window.limit}`;
+		if (window?.beforeSeq !== undefined) url += `&beforeSeq=${window.beforeSeq}`;
 		const res = await fetchImpl(url);
 		if (!res.ok) {
 			throw new Error(`History sync failed: ${res.status}`);