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

go.mod

@@ -5,20 +5,19 @@ go 1.25.0
 require (
 	connectrpc.com/connect v1.19.1
 	github.com/go-chi/chi/v5 v5.2.5
+	github.com/google/uuid v1.6.0
 	github.com/gorilla/websocket v1.5.3
 	github.com/jackc/pgx/v5 v5.8.0
 	github.com/vishvananda/netlink v1.1.1-0.20210330154013-f5de75959ad5
 	github.com/vishvananda/netns v0.0.0-20210104183010-2eb08e3e575f
+	golang.org/x/sys v0.42.0
 	google.golang.org/protobuf v1.36.11
 )
 
 require (
-	github.com/bits-and-blooms/bitset v1.24.4 // indirect
-	github.com/google/uuid v1.6.0 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
 	golang.org/x/sync v0.17.0 // indirect
-	golang.org/x/sys v0.42.0 // indirect
 	golang.org/x/text v0.29.0 // indirect
 )