[tire-fire]

Fixes #177.

Summary

• AttemptDelay: seconds the runner waits before retrying a failed attempt. It can be set per check or as an event-wide default in [MiscSettings]. The runner skips a retry that couldn't start before the round deadline, and stops waiting if it's shutting down.
• Attempts can now also be set event-wide in [MiscSettings]; a value set on a check still wins. Negative values are rejected.
• Config validation rejects any check whose worst case can't finish in the shortest round: Attempts * Timeout + (Attempts - 1) * AttemptDelay must be less than Delay - Jitter.
• README documents Attempts/AttemptDelay in [MiscSettings] and in the per-check property table (attempts wasn't documented before).
• Separate commit: stale entries in the Redis results queue are now cleared at round start along with stale tasks. After a mid-round restart the next round reuses the round ID, and leftover results from the interrupted round were being counted as the new round's.

With the defaults (Attempts = 1, AttemptDelay = 0) behavior is unchanged.

Tests

• engine/config/attempt_config_test.go: a rapid property test that a check is accepted exactly when its attempt budget fits in Delay - Jitter and receives the event defaults; per-check values aren't overridden; Attempts defaults to 1; negative values are rejected.
• runner/runner_test.go: waitForRetry waits the delay, skips when the deadline is too close, and returns on cancellation.
• engine/engine_test.go: TestRvb_IgnoresStaleResultsFromRestartedRound fails without the fix and passes with it.
• The full suite, go vet and golangci-lint pass.