feat(wire,conversation-store): per-chunk seq sync cursor (StoredChunk)

Add StoredChunk { seq, role, chunk } to @dispatch/wire (re-exported via the
kernel contract shims). Keeps Chunk pure (provider-facing, no cursor); the
sync cursor lives only on the envelope.

conversation-store: rekey conv:<id>:msg:<seq> -> conv:<id>:chunk:<seq>;
append explodes messages into role-tagged seq'd chunks (1-based, gap-free,
monotonic) with internal boundary metadata so load() round-trips ChatMessage[]
losslessly and still reconciles; new loadSince(id, sinceSeq?) raw sync stream.
session-orchestrator test fake conforms to the widened interface.

FE Slice 2 backend prereq (per-chunk seq). typecheck clean, 469 vitest, biome clean.

1 files changed, 16 insertions, 0 deletions

diff --git a/packages/wire/src/index.ts b/packages/wire/src/index.ts
index d2ea341..82fb3ed 100644
--- a/packages/wire/src/index.ts
+++ b/packages/wire/src/index.ts
@@ -92,6 +92,22 @@ export interface ChatMessage {
 	readonly chunks: readonly Chunk[];
 }
 
+/**
+ * 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 pure content unit, unchanged — `Chunk` itself never
+ * carries storage metadata (it is also passed to/from the provider, which has
+ * no use for a cursor).
+ */
+export interface StoredChunk {
+	readonly seq: number;
+	readonly role: Role;
+	readonly chunk: Chunk;
+}
+
 // ─── Usage ──────────────────────────────────────────────────────────────────
 
 /**