feat(55): stop leaving disk debts — temp clips and a low-disk guard

595 green, 0 warnings. Completes TASK 55 (slice 1, log rotation, shipped 2026-10-22).

- **`AlertClipCleanup`**: the default alert clip is materialised into `%TEMP%` for ffmpeg to read, and
  nothing ever deleted it — one orphan per profile that ever played an alert. Now: delete this session's
  file in `Shutdown`, and sweep `ytLive-alert-*.mp4` at startup for anything a crash left behind. Only
  the prefix we own is touched; a directory is never deleted.
- **`DiskSpaceGuard`**: a recording that dies of a full disk is a trust breach, so the numbers are a
  pure policy — refuse to start under **1 GB** (toast; the stream still starts, recording does not),
  warn under **5 GB** at start ("about eighty minutes" at 8 Mbps ≈ 3.6 GB/h), and one mid-show warning
  under **2 GB** — the last useful moment before the encoder fails for a reason it won't explain.
  Unreadable drives (a USB stick that just left) return null and never block a running recording.

Tests are policy-level on purpose: filling a real drive is not a test. `DiskHygieneTests` proves the
sweep only removes our own files and that the thresholds behave at both ends.

ViewModels/MainViewModel.Recording.cs

@@ -16,12 +16,32 @@ public partial class MainViewModel : ViewModelBase
     private DateTime _recordStartTime;
     private TimeSpan _recordLength;
 
+    private bool _lowDiskWarned;
+
     private string? StartRecordFile()
     {
         try
         {
             var folder = _recordFolder ?? DefaultRecordFolder();
             Directory.CreateDirectory(folder);
+
+            // TASK 55·3: never start something the drive cannot finish.
+            if (DiskSpaceGuard.FreeBytes(folder) is { } free)
+            {
+                var decision = DiskSpaceGuard.ForStart(free);
+                if (decision == DiskSpaceGuard.StartDecision.Block)
+                {
+                    AppLog.Write($"Recording refused: {free / (1024 * 1024)} MB free",
+                        AppLogMessageType.Error, AppLogErrorClass.Class2);
+                    _notifications.Error("Not enough disk space",
+                        "There isn't room to record safely. Free up some space or pick another record folder.");
+                    return null;
+                }
+                if (decision == DiskSpaceGuard.StartDecision.Warn)
+                    _notifications.Warning("Disk space is getting low",
+                        "Your recording starts, but there isn't much room left. A long show may not fit.");
+            }
+            _lowDiskWarned = false;
             var name = RecordingFile.BuildStartName(DateTime.Now) + ".mp4";
             var path = Path.Combine(folder, name);
             _activeRecordPath = path;