audit-log: #[must_use] on record() per review

argus: record() is pub and returns Option<AuditEntry>; the hive rule
requires #[must_use] on public fns returning a non-trivial value. Call
sites already consume the return (handle_restart_infra via if-let, tests
via let _).

hive-c0re/src/audit_log.rs

@@ -140,6 +140,7 @@ impl AuditLog {
     /// live event (e.g. the dashboard stream) emit *this* rather than
     /// re-deriving the fields, so the stored row and the streamed event
     /// can't drift.
+    #[must_use]
     pub fn record(
         &self,
         agent: &str,