docs(adr): a gateway dispatch is a probe, and the gateway records every fleet request's outcome (ADR 0021)

[V3RON]

ADR 0021 for #329. It replaces ADR 0016 §6's fleet join and narrows ADR 0014 §6. Task #431 implements it, and #347's usage rules now follow it.

• Probe. Every gateway dispatch carries fleetRequestId. Only the gateway's uplink may set it, and only with noWait. A worker never queues a probe.
• The worker declines, never rejects, a probe. Every refusal or failure of a probe is lease.declined, whatever the reason. The worker makes no judgement about whether the gateway will retry.
• The gateway records every outcome itself. It emits request.granted when it hands a grant to its caller. It emits lease.rejected for every other ending, with the new reason worker-failed (+ code, worker) for failures on a worker. A gateway stop or crash is closed on read, at its next daemon.started.
• Usage. Fleet outcomes and waits come from the gateway's own events and clock. Device facts come from relayed events, matched by worker and worker lease id. declined counts events. On a worker, probes are counted apart from local requests.
• Protocol +1. An older worker is incompatible.

Why: #347's review round 3 found that a stale worker's relayed no-wait refusal counted as a request's outcome. Four spec-check rounds on #431 shaped the rest.

Review

Spec review: 0 blocking, 0 fixed, 0 notes. Code review: skipped. Claims review: skipped.
Mutate: 0 mutants, 0 alive.

• Five spec-check rounds on A gateway dispatch is a probe: workers record lease.declined, the gateway records every fleet outcome #431 tested the design: 18, 10, 10, 7 and 7 findings, all folded into this ADR, A gateway dispatch is a probe: workers record lease.declined, the gateway records every fleet outcome #431 and usage.get and simlock stats: the figures from the event history, on a worker and a gateway #347. Per the maintainer, there is no separate review round on this docs-only PR.
• The maintainer accepted the ADR on 2026-10-09.

Refs #329, #431

Written by an agent.