feat: publish standalone worker

Separate worker packaging and service lifecycle from the control plane.

internal/webapp/metrics_linux.go

@@ -0,0 +1,29 @@
+//go:build linux
+
+package webapp
+
+import (
+	"syscall"
+)
+
+// statDisk fills Total/Free/Used for a DiskSample via statfs(2).
+// Linux-only because syscall.Statfs_t is platform-specific. Tests
+// that exercise the metric paths stub statDisk at the function
+// boundary so the build is hermetic.
+func statDisk(mount string, d *DiskSample) error {
+	var st syscall.Statfs_t
+	if err := syscall.Statfs(mount, &st); err != nil {
+		return err
+	}
+	bsize := uint64(st.Frsize)
+	total := st.Blocks * bsize
+	free := st.Bavail * bsize
+	used := total - free
+	d.Total = total
+	d.Free = free
+	d.Used = used
+	if total > 0 {
+		d.UsedPct = pct(used, total)
+	}
+	return nil
+}