feat(vm): replace Firecracker with Cloud Hypervisor

Migrate the entire VM layer from Firecracker to Cloud Hypervisor (CH).
CH provides native snapshot/restore via its HTTP API, eliminating the
need for custom UFFD handling, memfile processing, and snapshot header
management that Firecracker required.

Key changes:
- Remove fc.go, jailer.go (FC process management)
- Remove internal/uffd/ package (userfaultfd lazy page loading)
- Remove snapshot/header.go, mapping.go, memfile.go (FC snapshot format)
- Add ch.go (CH HTTP API client over Unix socket)
- Add process.go (CH process lifecycle with unshare+netns)
- Add chversion.go (CH version detection)
- Refactor sandbox manager: remove UFFD socket tracking, snapshot
  parent/diff chaining, FC-specific balloon logic; add crash watcher
- Simplify snapshot/local.go to CH's native snapshot format
- Update VM config: FirecrackerBin → VMMBin, new CH-specific fields
- Update envdclient, devicemapper, network for CH compatibility

internal/layout/layout.go

@@ -46,7 +46,7 @@ func SandboxesDir(wrennDir string) string {
 	return filepath.Join(wrennDir, "sandboxes")
 }
 
-// KernelPath returns the path to the Firecracker kernel.
+// KernelPath returns the path to the VM kernel.
 func KernelPath(wrennDir string) string {
 	return filepath.Join(wrennDir, "kernels", "vmlinux")
 }