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_files_stream.go

@@ -2,6 +2,7 @@ package api
 
 import (
 	"io"
+	"log/slog"
 	"mime"
 	"mime/multipart"
 	"net/http"
@@ -189,6 +190,6 @@ func (h *filesStreamHandler) StreamDownload(w http.ResponseWriter, r *http.Reque
 
 	if err := stream.Err(); err != nil {
 		// Headers already sent, nothing we can do but log.
-		// The client will see a truncated response.
+		slog.Warn("file stream error after headers sent", "error", err)
 	}
 }