feat: publish standalone worker

Separate worker packaging and service lifecycle from the control plane.

internal/webapp/metrics_other.go

@@ -0,0 +1,11 @@
+//go:build !linux
+
+package webapp
+
+// statDisk is a no-op on non-Linux platforms. The webapp MVP targets
+// Linux workers (the proc/sysfs data sources only exist there); the
+// build tag keeps the package compilable on a developer Mac for
+// quick template/handler iteration.
+func statDisk(_ string, _ *DiskSample) error {
+	return nil
+}