xmrig-auto-pause: fix stuck state after external restart, add startup cooldown

Two bugs found during live verification on the server:

1. Stuck state after external restart: if something else restarted xmrig
   (e.g. deploy-rs activation) while paused_by_us=True, the script never
   detected this and became permanently stuck — unable to stop xmrig on
   future load because it thought xmrig was already stopped.

   Fix: when paused_by_us=True and busy, check if xmrig is actually
   running. If so, reset paused_by_us=False and re-stop it.

2. Flapping on xmrig restart: RandomX dataset init takes ~3.7s of intense
   non-nice CPU, which the script detected as real workload and immediately
   re-stopped xmrig after every restart, creating a start-stop loop.

   Fix: add STARTUP_COOLDOWN (default 10s) — after starting xmrig, skip
   CPU checks until the cooldown expires.

Both bugs were present in production: the script had been stuck since
Apr 3 (2+ days) with xmrig running unmanaged alongside llama-server.

services/monero/xmrig-auto-pause.nix

@@ -26,6 +26,7 @@ lib.mkIf config.services.xmrig.enable {
       POLL_INTERVAL = "3";
       GRACE_PERIOD = "15";
       CPU_THRESHOLD = "5";
+      STARTUP_COOLDOWN = "10";
     };
   };
 }