Follow-up hardening for the audit control plane. The ABBA deadlock
(backlog#961), dispatch failure propagation (backlog#962), and credential
header redaction (backlog#963) already landed on main; this change addresses
the remaining audit-side findings.
- backlog#970: reload/commit now shuts down existing replay workers and closes
old targets *before* activating the replacement set, so old and new workers
never drain the same store concurrently and re-deliver entries. Extracted a
state-neutral `shutdown_runtime_targets` helper (registry-then-cancellers
lock order preserved).
- backlog#978: `start()` claims the `Starting` transition atomically under the
state lock, closing the check-then-act race that could double-activate;
`dispatch()` no longer returns Ok while paused, it surfaces an explicit
`AuditError::Paused` so the audit trail is not silently corrupted.
- backlog#984 (audit part): `dispatch_audit_log` performs a single state read
and interprets not-running/paused as a deliberate skip while surfacing real
delivery failures; `dispatch_batch` records the same observability signals as
single dispatch; documented the unordered cross-target fan-out.
Adds unit tests: paused dispatch returns Err, concurrent start does not hang or
double-activate, commit closes old targets before installing new, and
dispatch/dispatch_batch delivery-outcome coverage (all-fail -> Err, partial ->
Ok, all-success -> Ok).
Relates to rustfs/backlog#970
Relates to rustfs/backlog#978
Relates to rustfs/backlog#984
Co-authored-by: heihutu <[email protected]>
