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_snapshots.go

@@ -84,7 +84,9 @@ func (h *snapshotHandler) Create(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 		// Delete existing template record and files.
-		h.db.DeleteTemplate(ctx, req.Name)
+		if err := h.db.DeleteTemplate(ctx, req.Name); err != nil {
+			slog.Warn("failed to delete existing template", "name", req.Name, "error", err)
+		}
 	}
 
 	// Verify sandbox exists and is running or paused.