TASK 55 slice 1: the startup log is size-capped

The creator (2026-10-09): 'we need some sort of log-rotation so that we don't
inadvertently fill up a user's drive.' startup.log was uncapped - 23MB and growing
every few seconds, because FfmpegEncoder logs every non-progress ffmpeg stderr line.

The policy lives in a new Helpers/LogRotation.cs as a pure function over a path
(RollIfNeeded(logPath, maxBytes, retainedFiles)), so AppLog's 156 call sites keep
their surface and the rule is testable on its own. At 5MB the live file rolls to
startup.1.log and one rolled copy is kept - the FILE SET is bounded, not just the
live file, so a long stream cannot grow the log without limit. A failed rotation
degrades to the old behaviour instead of taking the app down.

Pattern reference (cited): Serilog's file sink pairs fileSizeLimitBytes (the cap)
with retainedFileCountLimit (how many rolled files to keep) -
https://github.com/serilog/serilog-sinks-file

Guard: ytLive.Tests/AppLogRotationTests.cs - the ONE test drives the real
AppLog.Write against a temp dir (two internal seams: PathOverride, MaxBytes) and
asserts the roll, the fresh live file, and that after a SECOND roll the directory
still holds exactly startup.log + startup.1.log.

Also fixed a stale catalog row: TASK 12 (master limiter) shipped 2026-08-15 and was
still marked 'Queued'.

TASKS.md

@@ -31,7 +31,7 @@
 | 09 | YouTube Live Stream Management | ⏳ In progress — items 1–3, 4 shipped; item 5 open; item 6 locked (TASK 36) | [`TASKS/task-09-live-stream-management.md`](TASKS/task-09-live-stream-management.md) |
 | 10 | Monetization: watermark-only paid delta (branding flash) | 🔶 **TEARDOWN, not a build** — Polar deleted in favour of Store IAP (2026-09-27); `$10/mo` + `$400` lifetime, no annual. **SUPERSEDED 2026-10-08** — free-forever ruling (watermark-free, gratuity/merch only); see TASK 54 | [`TASKS/task-10-monetization.md`](TASKS/task-10-monetization.md) |
 | 11 | Post-pause polish batch (creator's 8 review issues) | ✅ SHIPPED 2026-08-15 | [`TASKS/task-11-polish-batch.md`](TASKS/task-11-polish-batch.md) |
-| 12 | Master limiter on the live mix | ☐ Queued | [`TASKS/task-12-master-limiter.md`](TASKS/task-12-master-limiter.md) |
+| 12 | Master limiter on the live mix | ✅ Shipped 2026-08-15 — `Services/Audio/MasterLimiter.cs` (−1 dBFS ceiling, instant attack, smoothed release) applied at the end of `AudioMixer.FillAndMix` | [`TASKS/task-12-master-limiter.md`](TASKS/task-12-master-limiter.md) |
 | 13 | Social media launch kit | 🔶 Scoped — queued after v1 | [`TASKS/task-13-social-launch-kit.md`](TASKS/task-13-social-launch-kit.md) |
 | 14 | Creator feedback batch | 🔶 In progress — Branch 2 shipped | [`TASKS/task-14-creator-feedback-batch.md`](TASKS/task-14-creator-feedback-batch.md) |
 | 15 | Stock scene background images | ✅ Shipped | [`TASKS/task-15-scene-backgrounds.md`](TASKS/task-15-scene-backgrounds.md) |
@@ -70,7 +70,7 @@
 | 52 | **Zoom box (anchored 2× magnifier)** | ✅ Done — shipped 2026-10-08 (3 creator corrections during the build: loupe → anchored capture over the full composite, drawn last → never shows the dim shading) | [`TASKS/task-52-zoom-box.md`](TASKS/task-52-zoom-box.md) |
 | 53 | **Output-format verification: Shorts/TikTok geometry (9:16)** | ☐ Queued (2026-10-08) — output formats never tested end-to-end; pin + prove the vertical geometry | [`TASKS/task-53-output-formats.md`](TASKS/task-53-output-formats.md) |
 | 54 | **Marcom round: free-forever positioning + merch storefront** | 🔶 In progress — llamacasty.com copy live 2026-10-08 (pricing stripped); merch POD + app About links queued | [`TASKS/task-54-marcom-free-merch.md`](TASKS/task-54-marcom-free-merch.md) |
-| 55 | **Disk hygiene: log rotation + low-disk guard** | ☐ Queued (2026-10-09) — `startup.log` appears uncapped; also temp alert-clip cleanup + free-space check before recording | [`TASKS/task-55-disk-hygiene.md`](TASKS/task-55-disk-hygiene.md) |
+| 55 | **Disk hygiene: log rotation + low-disk guard** | 🔶 In progress — **slice 1 shipped 2026-10-22** (`startup.log` rolls at 5 MB to `startup.1.log`, one copy kept — it was uncapped at 23 MB and growing every few seconds); slices 2 (temp alert-clip cleanup) + 3 (low-disk guard) open | [`TASKS/task-55-disk-hygiene.md`](TASKS/task-55-disk-hygiene.md) |
 | 56 | **Pre-publication secret scan (full git history)** | ✅ Done — 2026-10-22, **CLEAN** (432 reachable commits + 63 dangling + worktree; 4 hits, all false positives: a `your-client-secret` doc placeholder and a test dummy — nothing to rotate). ⚠️ the repo was **already public** when the scan ran (anonymous GET → 200), so the gate came late — but nothing was exposed | [`TASKS/task-56-secret-scan.md`](TASKS/task-56-secret-scan.md) |
 | 57 | **Vertical (9:16) tier camera placement — the "vv" feature** | ✅ **Both units shipped 2026-10-21** (Unit 1 = Live scene: own placement per tier — small tile, circle-or-rect, one of six spots; drag-snap + manual entry; 16:9 restored exactly. Unit 2 = Chat scene: rect only — the circle is not offered — the tile or one whole 607×540 area) | [`TASKS/task-57-vertical-camera-layout.md`](TASKS/task-57-vertical-camera-layout.md) |
 | 58 | **Properties-panel style consistency (the theme owns the look)** | ✅ Done — 2026-10-21 (the creator's "why is the drop-down not in the same style as the others?" — the drop-downs/fields/check boxes had been hand-painted instead of themed, so WPF drew the light system template; now every one carries a theme style, guarded by a test) | [`TASKS/task-58-properties-style-consistency.md`](TASKS/task-58-properties-style-consistency.md) |