feat(media): add a video file layer that decodes into the scene

TASK 21's picker slice: the decoder, manager, pacing and loop mechanism were
already shipped but nothing ever acquired a session, so a media Source rendered
nothing. Adds "Video File" to the (+) menu (video-filtered OpenFileDialog),
one decode session per distinct path with a HasSession guard, release when the
last scene reference goes, and a post-load reconcile. Loop is a manager-level
per-path answer ("any live reference loops") applied before StartAsync and
updated live via SetLooping. Properties panel shows the file with Change… and
Loop. Volume stays unwired: media audio rides the desktop loopback, TRAX-style.

Reference: OBS's media source (file → decode → composite into the scene) is the
established answer for this feature; the lifecycle shape mirrors this repo's
WebView2/ScreenCapture registration rather than inventing a new one.

Test: ytLive.Tests/MediaSourceAcquisitionTests.cs — add starts one decoder and
frames reach the layer, loop flip reaches the decoder, remove stops it.

Docs: task-21 closed, TASKS row + Day-4 row + stale 3·17 row corrected,
frame-pipeline acquisition note, HANDOFF rewritten. verify.sh green.

TASKS.md

@@ -20,7 +20,7 @@ corrected. No new scope mid-week; anything discovered goes on a list for day 6.
 | **1 · Mon** — licensing | Paid-app ownership check (`BIsSubscribedApp`) + Debug fake + app-wide `IsAppOwned` · greyed tier entries · About conversion copy + llamacasty.com link · current-launch Logs tab (replaces the separate system-message strip) · Debug-only `InternalsVisibleTo` | A locked tier opens About without changing the selected free tier or leaving the app; the site opens only from its link |
 | **2 · Tue** — release blockers | Visibility flow (creator-selected at release; development/test stays Private) · `OutputProfile` entitlement: 30 fps free / 60 fps paid regardless of resolution · ~~`StreamConfig` defaults~~ retired (no production call sites) · TASK 53 (vertical resolution check ✅; remaining output contract open) · 9·5 webcam identity key · Move transitions (per-layer carry-across-scenes) | Visibility is selectable at creation and live-editable; the 30/60-fps gate is tested; remaining Day 2 items stay queued |
 | **3 · Wed** — resilience | TASK 32: blip retry → measured grace → amber RECONNECTING → one-click Back on air → fragmented MP4 → ingest pre-flight · 33 bandwidth step-down | Killing the network mid-broadcast recovers without ending it |
-| **4 · Thu** — content & media | 3·17 Text layer · 21 media acquisition wiring + file picker + loop/volume UI · 49 chat profanity filter · 34 scheduled streams | Text layer renders/persists; media file plays, loops, has volume; scheduled stream created |
+| **4 · Thu** — content & media | ~~3·17 Text layer~~ ✅ · ~~21 media acquisition wiring~~ ✅ (picker, loop, lifecycle) · 49 chat profanity filter · 34 scheduled streams | Text layer renders/persists; media file plays, loops; scheduled stream created |
 | **5 · Fri** — data & settings | 3·20 RewardEvent persistence + `superChatEvents.list` backfill · 39·2 YPP slice 2 · 37 defaults-vs-current split · 35 scene-linked audio · 40·A/C/D settings round · 55·2–3 disk hygiene | Session report reads persisted events; defaults can be reverted; temp clips cleaned |
 | **6 · Sat** — finish & prove | 50 publish recording · preview undo (Ctrl+Z) · 14 creator feedback batch · 36 gold pass · full sweep: clean build, whole per-class suite, `publish.sh` artifact, EULA/privacy/notices in the artifact · commit + push | Gold pass list empty; artifact stages; suite green; pushed |
 
@@ -29,11 +29,9 @@ in the middle.
 
 ## Next up (after the surge slice)
 
-**TASK 3·17 — the Text layer** is scheduled in the surge (day 4). Placeholder detail retained for
-the unit itself: `AddSource` creates it 0×0, all four compositor paths `continue` past it,
-`FramePump`'s hash skips it, it is not draggable, and it has no text/font/colour properties or
-persistence. Text content, font family/size/weight/style, colour, alignment, drag/resize,
-persisted; renderer mirrors `Services/ChatBoxRenderer.cs`.
+**Day 4 remainder:** TASK 49 (chat profanity filter) then TASK 34 (scheduled streams — its two research
+flags must be cited first). Day 3's resilience row (TASK 32/33) is still outstanding and is a launch
+blocker; the smallest slice is 32·4 (fragmented MP4 on the record block).
 
 ## Queue
 
@@ -43,8 +41,8 @@ persisted; renderer mirrors `Services/ChatBoxRenderer.cs`.
 |---|---|---|---|
 | 53 | ✅ | Output-format contract — six size/FPS modes, vertical YouTube playback, and recordings visually accepted | [`task-53`](TASKS/task-53-output-formats.md) |
 | — | ✅ | Output entitlement code: free at 30 fps / paid at 60 fps regardless of resolution; Steam App ID is still pending | [`commercial`](docs/ai/commercial.md) |
-| 3·17 | ☐ **NEXT** | **Text layer** — content, font family/size/weight/style, colour, alignment, drag/resize, persisted | [`task-03`](TASKS/task-03-capture-pipeline.md) |
-| 21 | 🔶 | Media source: acquisition wiring + file picker + loop/volume UI (decoder, manager, pacing, loop all shipped — **no `AcquireAsync` caller exists**) | [`task-21`](TASKS/task-21-media-source.md) |
+| 3·17 | ✅ | **Text layer** — content, font family/size/weight/style, colour, alignment, drag/resize, persisted (done 2026-11-04) | [`task-03`](TASKS/task-03-capture-pipeline.md) |
+| 21 | ✅ | Media source: video file added from the (+) menu decodes, composites, loops, and stops with its last reference (done 2026-11-05) | [`task-21`](TASKS/task-21-media-source.md) |
 | — | ☐ | Transitions **Move**: per-layer carry-across-scenes flag, per-scene placement, fixed z band | [`ui-surface`](docs/ai/ui-surface.md) |
 | 55·2–3 | ☐ | Disk hygiene: delete `%TEMP%` alert clips on close + sweep at startup; low-disk guard before REC (slice 1 shipped) | [`task-55`](TASKS/task-55-disk-hygiene.md) |
 | 40·A/C/D | ☐ | App Settings: active-camera picker; persist output resolution + surface transition defaults + confirm-before-End; accent consolidation | [`task-40`](TASKS/task-40-app-settings-round.md) |