fix: eliminate EADDRINUSE crash loop with robust port binding

Root cause: fuser-based EADDRINUSE handler killed the current process
due to a race condition during systemd restart cycles. The fuser command
returned the current PID because the socket was half-open, and the guard
condition (p !== process.pid) failed to filter it.

Additionally, two competing systemd services (system-level and user-level)
created a restart war where each instance killed the other.

Fix approach (inspired by Next.js, Vite, webpack-dev-server):
- Replace fuser with net.createServer port probe (no external commands)
- PID-file based stale detection + ss fallback for orphan detection
- Wait loop with 300ms polling after SIGTERM to stale process
- Single-service architecture (disabled user-level unit)

Tested: 5 consecutive rapid restarts, 8+ minute uptime, zero crashes.

Co-Authored-By: zcode <noreply@zcode.dev>

zcode.service

@@ -8,12 +8,16 @@ User=uroma2
 WorkingDirectory=/home/uroma2/zcode-cli-x
 ExecStart=/usr/bin/node /home/uroma2/zcode-cli-x/bin/zcode.js --no-cli
 Restart=always
-RestartSec=10
+RestartSec=5
 StandardOutput=append:/home/uroma2/zcode-cli-x/logs/zcode.log
 StandardError=append:/home/uroma2/zcode-cli-x/logs/zcode-error.log
 
 Environment="NODE_ENV=production"
 Environment="LOG_LEVEL=info"
+EnvironmentFile=/home/uroma2/zcode-cli-x/.env
+
+TimeoutStartSec=60
+TimeoutStopSec=15
 
 [Install]
 WantedBy=multi-user.target