feat(fleet): fan-in census line + engine-shaped stream fixtures #1771

Acceptance Criteria

• A fan-in service logs a census line every 30 s with: active listeners; listeners accepted and closed since boot; evictions; and per arm (local and each peer) frames, bytes, largest frame, time since last frame, opens, closes by reason, pending depth, overflow drops, and gaps emitted.
• Close reasons are an enumerated set: upstream end, upstream error, membership change, listener closed, evicted, shutdown.
• Each transition (listener accepted or closed, arm opened or closed, gap emitted, overflow drop) logs one line immediately, with the connection and arm identity.
• Each ownership refresh logs, per peer, the outcome (ok, timeout, error, unauthorized), its duration, and its entry count — a failed peer refresh is visible even though the feed never throws.
• The census adds no frames to any stream and changes no delivery behavior; existing fan-in suites pass with only fixture updates.
• Engine-shaped fixtures exist and the fan-in suites use them: id-less data frames, heartbeat frames, the plain-plus-sync twin emission of a stored event, and a 10 MB frame delivered in 64 KB chunks.

Testing Decisions

• Extend the fan-in aggregator and driver pump suites to assert counters after scripted sequences, and the owner-map feed tests to assert a per-peer outcome record for a failed refresh.
• The engine-shaped fixtures are new shared test surface; the loud-gap slice and the linear-splitting slice build on them.

Key Decisions

• Census reasons are enumerated so later slices and the verification slice can assert on them.
• Cadence: 30 s steady state, transitions immediately, never per frame.

Constraints & Invariants

• Zero wire changes; fleet-of-one byte identity (Fleet thin client — Slice 4: lossless SSE reconnect (cursor resume) #1264) holds.

Prior Art

• The fan-in driver's connection and arm lifecycle and its pump; the aggregator's flowing, pending, drop, and gap state; the owner-map feed's refresh loop and its never-throw projection builder; the hub runner's log and the window's Amicode output channel.

Source

• Part of Fleet event stream is lossy by construction — remote sessions lag and desync during live activity #1770.