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, 19 insertions, 7 deletions

diff --git a/.dispatch/wire.reference.md b/.dispatch/wire.reference.md
index 40f94cf..1d761bf 100644
--- a/.dispatch/wire.reference.md
+++ b/.dispatch/wire.reference.md
@@ -4,8 +4,14 @@
 > types WITHOUT following the `file:` dep symlink out of this repo (which hangs on a permission
 > prompt). Your CODE still imports `@dispatch/wire` normally — this file is for READING only.
 >
-> **Orchestrator:** SNAPSHOT of `[email protected]` (the metrics types below + the `user-message` turn event
-> shipped + version-bumped). Regenerate whenever `@dispatch/wire` changes.
+> **Orchestrator:** SNAPSHOT of `[email protected]` (doc-only bump: the 1-based gap-free seq guarantee
+> codified on `StoredChunk`). Regenerate whenever `@dispatch/wire` changes.
+>
+> **2026-06-12 delta (CR-5 history windowing — package bumped `0.6.0` → `0.6.1`, DOC-ONLY):** the
+> per-conversation `seq` numbering is now a WRITTEN CONTRACTUAL GUARANTEE on `StoredChunk`:
+> **1-based, monotonic, gap-free** — a conversation's first chunk is always `seq === 1` and
+> numbering never skips. A client holding only a windowed suffix of the log derives "older chunks
+> exist server-side" purely from `oldestLoaded.seq > 1` (no `earliestSeq`/`hasOlder` field exists).
 >
 > **2026-06-12 delta (CR-3 user-message handoff — package bumped `0.5.0` → `0.6.0`, ADDITIVE):** adds a
 > new `AgentEvent` union member `TurnInputEvent` (`{ type: "user-message"; conversationId; turnId; text }`)
@@ -168,11 +174,17 @@ export interface ChatMessage {
 
 /**
  * A persisted chunk plus its sync metadata. The append-only conversation log
- * stamps every chunk with a monotonic, gap-free, per-conversation `seq` (the
- * sync cursor, assigned in append order) and records the `role` of the message
- * it belongs to. This makes a flat seq-ordered stream both incrementally
- * syncable ("give me chunks after seq N") and regroupable into messages by the
- * client. `chunk` is the content unit — `Chunk` carries no storage/sync cursor
+ * stamps every chunk with a **1-based**, monotonic, gap-free, per-conversation
+ * `seq` (the sync cursor, assigned in append order) and records the `role` of
+ * the message it belongs to. This makes a flat seq-ordered stream both
+ * incrementally syncable ("give me chunks after seq N") and regroupable into
+ * messages by the client.
+ *
+ * The 1-based start is a CONTRACTUAL GUARANTEE (not an implementation detail):
+ * a conversation's first chunk is always `seq === 1` and numbering never skips,
+ * so a client holding only a windowed suffix of the log can derive "older
+ * chunks exist server-side" purely from `oldestLoaded.seq > 1` — no separate
+ * has-older flag is needed (or provided). `chunk` is the content unit — `Chunk` carries no storage/sync cursor
  * (`seq` lives here on the envelope, not on the chunk, since it is assigned by
  * the store and the provider has no use for it). A chunk MAY still carry
  * generation provenance assigned at production time (e.g. a tool chunk's