feat(acp): per-turn stage latency instrumentation (#2386)

[brocoppler]

What this solves

First slice of #2386: stage-level timing for the mention-to-reply path, keyed by the existing turn_id. Answers "which stage was slow" for stages 1–5 of the issue's boundary list.

Stages measured

• relayLagSecs — relay accept → harness receipt (wall clock, 1s resolution, from event created_at)
• admissionMs — receipt → queue admission (gates: owner-command, author, rule match, follow policy)
• queueWaitMs — admission → turn dispatch (oldest batch event)
• sessionSetupMs + sessionReused — dispatch → ACP session resolved; warm/cold split per the issue's benchmark axes
• firstOutputMs — prompt write → first agent_message_chunk
• turnTotalMs — dispatch → turn completion

All monotonic-clock except relay lag; all optional; no message content in any telemetry.

Carriers

1. kind:44200 AgentTurnMetricPayload — new optional camelCase fields (skip_serializing_if, backward-compatible; consumers already MUST ignore unknown fields). NIP-AM.md updated.
2. turn_completed observer frame — carries the same timings instead of {}.
3. One tracing::info! summary line per turn (pool::metrics, keyed by turn_id, outcome + *_ms fields) for log-based analysis.

Out of scope, per the issue's own split

• Stages 7–8 (agent publishes reply → relay OK) are architecturally unobservable from the harness today: replies go out-of-band via the agent's own CLI. buzz-acp: turn completes ok but reply is silently lost when the agent's buzz CLI write fails #2459 tracks that observability gap; these fields slot in once it lands.
• The repeatable benchmark harness, p50/p95 reporting, and CI budget gates — this PR provides the per-turn measurements those consume.

Testing

cargo test -p buzz-acp and -p buzz-core green (new tests: queue field threading through flush/requeue, timing finalization at turn exit, first-output mark/drain, payload serde round-trip incl. omission); clippy -D warnings clean across both crates; fmt clean; workspace check green.

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