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>

scripts/windows-vm.sh

@@ -69,16 +69,15 @@ Remove-Item "$PSScriptRoot\input.txt"'
 pointer() {  # x y click|wheel [notches]
   local b64=$(print -rn -- $INPUT_PS1 | base64 | tr -d '\n')
   vm_ps '$ErrorActionPreference = "Stop"
-$d = Join-Path $env:LOCALAPPDATA "windows-vm"; $req = "$d\input.txt"; $held = $false
-trap { [Console]::Error.WriteLine("windows-vm: $_"); if ($held) { Remove-Item $req -ErrorAction SilentlyContinue }; exit 1 }
+$d = Join-Path $env:LOCALAPPDATA "windows-vm"; $req = "$d\input.txt"; $mine = $false
+trap { [Console]::Error.WriteLine("windows-vm: $_"); if ($mine) { Remove-Item $req -ErrorAction SilentlyContinue }; exit 1 }
+# One request at a time: the helper, the task and input.txt are shared. Windows
+# frees the mutex when this process ends, however it ends.
+$lock = New-Object Threading.Mutex($false, "windows-vm-input")
+try { $mine = $lock.WaitOne(15000) } catch [Threading.AbandonedMutexException] { $mine = $true }
+if (-not $mine) { [Console]::Error.WriteLine("windows-vm: another click or scroll is still in progress"); exit 1 }
 New-Item -ItemType Directory -Force $d | Out-Null
-$old = Get-Item $req -ErrorAction SilentlyContinue   # left behind by a run that died
-if ($old -and $old.LastWriteTime -lt (Get-Date).AddSeconds(-15)) { Remove-Item $req }
-# Creating input.txt is the lock: CreateNew fails if another request already has it.
-try { $f = [IO.File]::Open($req, "CreateNew", "Write") }
-catch { [Console]::Error.WriteLine("windows-vm: another click or scroll is in progress"); exit 1 }
-$held = $true
-$text = [Text.Encoding]::ASCII.GetBytes("'"$*"'"); $f.Write($text, 0, $text.Length); $f.Close()
+[IO.File]::WriteAllText($req, "'"$*"'")
 [IO.File]::WriteAllText("$d\input.ps1", [Text.Encoding]::UTF8.GetString([Convert]::FromBase64String("'$b64'")))
 $act = New-ScheduledTaskAction -Execute powershell.exe -Argument "-NoProfile -WindowStyle Hidden -ExecutionPolicy Bypass -File `"$d\input.ps1`""
 $who = New-ScheduledTaskPrincipal -UserId $env:USERNAME -LogonType Interactive