fix(fleet): lossless resume end to end — relays and client resume from engine ids #1777

Acceptance Criteria

• A peer arm that drops and re-opens within the replay window resumes from its last delivered id, and the viewer receives every missed frame — no gap signal, no full resync.
• An upstream reset on any arm produces exactly one gap signal for that namespace downstream, and the client fully resyncs it.
• In fleet-of-one mode the listener's stream is byte-identical to the engine's, ids included.
• After a reconnect or a reload, the client resumes with the SSE id (composite when fanned in), and a sequence hole within one origin triggers a resync.
• Replayed frames are never applied twice: the client drops any frame whose sequence it has already applied.
• Two-peer fleet E2E: killing the peer link mid-turn and restoring it leaves the observer's transcript identical to a fresh load.

Testing Decisions

• Extend the aggregator and driver suites with epoch-and-sequence fixtures (replacing the id-less fixtures where ids now exist), the app's server-sdk and stream suites (cursor source, reset, hole, duplicate), and the slow two-peer fleet E2E (link kill). Extends the loud-gap slice's and the engine slice's test surface.

Key Decisions

• Contiguity is checked per origin namespace; heartbeats count because they carry the sequence, so a hole is detectable even on an idle stream.
• The client's existing session-storage cursor persists the SSE id.

Constraints & Invariants

• Fleet thin client — Slice 4: lossless SSE reconnect (cursor resume) #1264, Fan-in relay never re-arms backpressure (flowing stuck false) → session transcript + rail go stale until reload #1617, and Fan-in relay resumes re-opened arm from connect-time cursor + no gap frame on reconnect → silent event loss #1638 are preserved.

Prior Art

• The aggregator's composite cursor and per-namespace cursor accessor (Fan-in relay resumes re-opened arm from connect-time cursor + no gap frame on reconnect → silent event loss #1638); the client's session-storage cursor and origin resolution (SSE fan-in contaminates local directory store with remote sessions, dropping fleet badge #1539); the SDK stream reader's last-event-id handling.

Source

• Part of Fleet event stream is lossy by construction — remote sessions lag and desync during live activity #1770 · Blocked by fix(fleet): every lost stretch of events forces a resync — no silent gaps #1773, feat(engine): sequenced, replayable global event stream (epoch:seq ids, replay buffer, reset) #1776.