windows-vm.sh: serialise clicks with a named mutex instead of a lock file

Windows releases the mutex when the request's process ends, so there's no
stale lock to recover and no window where two requests can take each
other's. Requests sent together now run one after another.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/testing.md

@@ -205,9 +205,9 @@ scripts/windows-vm.sh down                       # shut Windows down
 - **Clicks go through a scheduled task.** Commands over SSH run in a
   session with no desktop, so `click` and `scroll` write the position to a
   file, and a scheduled task running as the signed-in user replays it. The
-  VM's screen must be signed in; it is after `up`. One click or scroll at a
-  time: they share that task. QEMU's own `mouse_move` is relative and drifts,
-  so the script doesn't use it.
+  VM's screen must be signed in; it is after `up`. Clicks and scrolls sent at
+  the same time run one after another. QEMU's own `mouse_move` is relative
+  and drifts, so the script doesn't use it.
 - **Screenshots may not show the pointer.** Check the result of a click (a
   menu that opens, a button that changes) rather than the pointer's position.
 - **Windows' `ssh` waits for stdin.** The script closes it for every command.