daemon: cut vm stop latency

Three changes to stopVMLocked, biggest win first:

- Skip waitForExit on the SSH-success path. sync inside the guest
  already flushed root.ext4, so cleanupRuntime's SIGKILL is safe
  immediately. Saves up to gracefulShutdownWait (10s) per stop.
- Drop the SendCtrlAltDel + 10s wait fallback when SSH is
  unreachable. On Debian, ctrl+alt+del routes to reboot.target so
  FC never exits on it — the wait was pure latency.
- Shrink the SSH dial timeout 5s → 2s. A reachable guest dials in
  single-digit milliseconds; if it doesn't, fail fast and SIGKILL.

Worst-case (broken SSH) goes ~15s → ~2s + cleanup.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/daemon/vm_test.go

@@ -1592,7 +1592,7 @@ func TestDeleteStoppedNATVMDoesNotFailWithoutTapDevice(t *testing.T) {
 	}
 }
 
-func TestStopVMFallsBackToForcedCleanupAfterGracefulTimeout(t *testing.T) {
+func TestStopVMSIGKILLsWhenSSHUnreachable(t *testing.T) {
 	ctx := context.Background()
 	db := openDaemonStore(t)
 	apiSock := filepath.Join(t.TempDir(), "fc.sock")
@@ -1606,12 +1606,6 @@ func TestStopVMFallsBackToForcedCleanupAfterGracefulTimeout(t *testing.T) {
 		}
 	})
 
-	oldGracefulWait := gracefulShutdownWait
-	gracefulShutdownWait = 50 * time.Millisecond
-	t.Cleanup(func() {
-		gracefulShutdownWait = oldGracefulWait
-	})
-
 	vm := testVM("stubborn", "image-stubborn", "172.16.0.23")
 	vm.State = model.VMStateRunning
 	vm.Runtime.State = model.VMStateRunning
@@ -1622,8 +1616,6 @@ func TestStopVMFallsBackToForcedCleanupAfterGracefulTimeout(t *testing.T) {
 		scriptedRunner: &scriptedRunner{
 			t: t,
 			steps: []runnerStep{
-				sudoStep("", nil, "chmod", "600", apiSock),
-				sudoStep("", nil, "chown", "-h", fmt.Sprintf("%d:%d", os.Getuid(), os.Getgid()), apiSock),
 				{call: runnerCall{name: "pgrep", args: []string{"-n", "-f", apiSock}}, out: []byte(strconv.Itoa(fake.Process.Pid) + "\n")},
 				sudoStep("", nil, "kill", "-KILL", strconv.Itoa(fake.Process.Pid)),
 			},