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.
gramps