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.

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