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.Streaming.Operations.cs

@@ -641,6 +641,16 @@ public partial class MainViewModel : ViewModelBase
         _recordLength = _liveElapsed;
         _recDotPulse = _recDotPulse > 0.5 ? 0.35 : 1.0;
         OnPropertyChanged(nameof(RecDotOpacity));
+
+        // TASK 55·3: the last useful warning before the encoder fails for a reason it won't explain.
+        if (IsRecording && !_lowDiskWarned && _recordFolder is { } folder
+            && DiskSpaceGuard.FreeBytes(folder) is { } free
+            && DiskSpaceGuard.ShouldWarnDuring(free))
+        {
+            _lowDiskWarned = true;
+            _notifications.Warning("Disk space is nearly gone",
+                "The recording may stop on its own. Free up space now, or stop and save what you have.");
+        }
     }
 
     /// <summary>What happened to a finished broadcast at the keep-or-delete prompt.</summary>