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/api/handlers_exec_stream.go

@@ -37,11 +37,6 @@ type wsStartMsg struct {
 	Args []string `json:"args"`
 }
 
-// wsStopMsg is sent by the client to stop the process.
-type wsStopMsg struct {
-	Type string `json:"type"` // "stop"
-}
-
 // wsOutMsg is sent by the server for process events.
 type wsOutMsg struct {
 	Type     string `json:"type"`                // "start", "stdout", "stderr", "exit", "error"