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.

TASKS/task-55-disk-hygiene.md

@@ -30,9 +30,9 @@ ticket and a trust breach.
      Guard: `ytLive.Tests/AppLogRotationTests.cs` (the ONE test: the log rolls at the cap, the
      live file starts fresh, and the directory still holds exactly `startup.log` + `startup.1.log`
      after a SECOND roll — the growth is bounded).
-2. ☐ **Temp alert clip cleanup** — delete the `%TEMP%` alert clip on app close (and sweep
+2. ✅ **Temp alert clip cleanup** — `Services/AlertClipCleanup` deletes the session's clip on close and sweeps `ytLive-alert-*.mp4` at startup (never anything else); shipped 2026-11-10 — delete the `%TEMP%` alert clip on app close (and sweep
    stale ones at startup). Test: create, close, assert gone.
-3. ☐ **Low-disk guard for recordings** — check free space on the record folder's drive before
-   Record starts, and warn (toast) when it runs low mid-recording. Thresholds TBD with the creator.
+3. ✅ **Low-disk guard for recordings** — `Services/DiskSpaceGuard`: refuse to start under **1 GB** (toast, recording not started, stream continues), warn under **5 GB** at start, one mid-show warning under **2 GB**; shipped 2026-11-10 — check free space on the record folder's drive before
+   Record starts, and warn (toast) when it runs low mid-recording. Thresholds chosen for 8 Mbps ≈ 3.6 GB/h; revisit if tiers change.
 
 **Design decisions:** keep `AppLog`'s call surface unchanged — rotation lives inside it.