Add device-mapper snapshots, test UI, fix pause ordering and lint errors

- Replace reflink rootfs copy with device-mapper snapshots (shared
  read-only loop device per base template, per-sandbox sparse CoW file)
- Add devicemapper package with create/restore/remove/flatten operations
  and refcounted LoopRegistry for base image loop devices
- Fix pause ordering: destroy VM before removing dm-snapshot to avoid
  "device busy" error (FC must release the dm device first)
- Add test UI at GET /test for sandbox lifecycle management (create,
  pause, resume, destroy, exec, snapshot create/list/delete)
- Fix DirSize to report actual disk usage (stat.Blocks * 512) instead
  of apparent size, so sparse CoW files report correctly
- Add timing logs to pause flow for performance diagnostics
- Fix all lint errors across api, network, vm, uffd, and sandbox packages
- Remove obsolete internal/filesystem package (replaced by devicemapper)
- Update CLAUDE.md with device-mapper architecture documentation

internal/uffd/server.go

@@ -115,7 +115,7 @@ func (s *Server) Ready() <-chan struct{} {
 // Stop signals the UFFD poll loop to exit and waits for it to finish.
 func (s *Server) Stop() error {
 	// Write a byte to the exit pipe to wake the poll loop.
-	s.exitW.Write([]byte{0})
+	_, _ = s.exitW.Write([]byte{0})
 	<-s.doneCh
 	return s.doneErr
 }