feat: add iowait, TCP connection states, and file descriptor metrics

- cpu.iowait_percent: % of time CPU is waiting on disk I/O
- tcp: established/time_wait/close_wait/listen counts; max_time_wait
  system limit included on Linux for context
- file_descriptors: used vs max (Linux reads /proc/sys/fs/file-nr,
  macOS reads kern.openfiles / kern.maxfiles via sysctl)

These three metrics directly surface the most common causes of
unexplained PHP/Laravel timeouts at the system level.

payload.json

@@ -21,6 +21,7 @@
   },
   "cpu": {
     "percent": 34.5,
+    "iowait_percent": 4.2,
     "per_core": [
       42.1,
       28.3,
@@ -74,6 +75,17 @@
     "packets_sent_per_sec": 95.3,
     "packets_recv_per_sec": 140.7
   },
+  "tcp": {
+    "established": 42,
+    "time_wait": 18,
+    "close_wait": 3,
+    "listen": 8,
+    "max_time_wait": 32768
+  },
+  "file_descriptors": {
+    "used": 1248,
+    "max": 98304
+  },
   "processes": [
     {
       "pid": 1234,