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/vm/config.go

@@ -10,8 +10,8 @@ type VMConfig struct {
 	// KernelPath is the path to the uncompressed Linux kernel (vmlinux).
 	KernelPath string
 
-	// RootfsPath is the path to the ext4 rootfs image for this sandbox.
-	// This should be a per-sandbox copy (reflink clone of the base image).
+	// RootfsPath is the path to the rootfs block device for this sandbox.
+	// Typically a dm-snapshot device (e.g., /dev/mapper/wrenn-sb-a1b2c3d4).
 	RootfsPath string
 
 	// VCPUs is the number of virtual CPUs to allocate (default: 1).