Stage 2: synchronize composite read state

[fujibee]

Summary

This stacked dogfood PR begins Stage 2 read-state synchronization. It defines the composite local/remote frontier and bounded exact-read protocol, then integrates the store-owned local cursor across inbox and monitor delivery.

The remote server and polling-engine implementation is intentionally deferred until the protocol review on ADR 0008 is clear. Stage 3 SSE/wake delivery is explicitly out of scope.

Contract

• Keep local positions store-local and merge only remote server frontiers across machines.
• Preserve out-of-order reads as exact local IDs, atomically promoting them to wire IDs with the durable message mapping.
• Keep transport, quarantine/decrypt, projection, and read progress independent.
• Bound and paginate exact remote state; garbage-collect it only from a proven wire-to-sequence mapping.
• Bind remote read state to immutable server/team/protocol/member identities.

Local integration

• Add per-pair read cursors to SQLite and JSONL.
• Share one cursor between inbox and monitor instead of per-session watermarks.
• Advance only through a contiguous covered prefix while retaining exact exceptions.
• Treat pre-cursor backlog as consumed during one-time migration to avoid an upgrade redelivery storm.

Verification

• SQLite storage contract: 32/32 passed.
• JSONL storage contract: 32/32 passed.
• Watch suite: 16/16 passed and exits cleanly.
• Inbox/check-inbox suite: 4/4 passed.
• Shell syntax and diff checks passed.

This PR must remain Draft and unmerged while the dogfood stack is under review.