quest: settle keyframes; plan a media file source and stinger transitions

quest/m0/README.md

@@ -93,7 +93,9 @@ repository.
 - [Audio source tracks](/quest/m0/audio-source-tracks.md) - microphones and system audio get their own tracks, for isolated recording and replay
 - [Broadcast sources play in sync](/quest/m0/broadcast-sync.md) - subscribed broadcasts and guests play audio and video on the publisher's timestamps, with `moq play`'s delay
 - [Wayland display and window sources](/quest/m0/wayland-sources.md) - portal-picked sources that reopen without the picker
-- [Keyframes and stingers](/quest/m0/animation.md) - keyframed item properties with easing, and stinger transitions
+- [Keyframes](/quest/m0/animation.md) - keyframed item properties with easing, starting when their scene goes on program
+- [Media file source](/quest/m0/media-file.md) - a local video or audio file plays as a source, looping or from the start on demand
+- [Stinger transitions](/quest/m0/stinger.md) - a clip played over the program that switches scenes at its cut point
 - [ML video effects](/quest/m0/effects.md) - background removal and other ML effects, baked into their source's track
 - [Destinations](/quest/m0/destinations.md) - RTMP, RTMPS, and WHIP push with per-destination codecs and shared encoders, on a UI-free egress library a hosted service can reuse
 - [Multiple canvases](/quest/m0/multi-canvas.md) - landscape and portrait canvases composite at once from shared sources

quest/m0/animation.md

@@ -1,39 +1,35 @@
-# [M] Keyframes and stingers
+# [M] Keyframes
 
 ## Goal
 
 Any item property in `moq-studio` can be keyframed with easing, so an item
-can fly in or a camera can push in, and a scene change can run a stinger: a
-video clip played over the program that switches scenes at its cut point.
+can fly in or a camera can push in, and the same animation plays in the
+program, a recording, and a replay.
 
 ## Plan
 
 Fade, slide, wipe, and move transitions are drawn by moq-scene's compositor
 on the program clock
 ([moq-dev/studio#33](https://github.com/moq-dev/studio/pull/33)). Animate in
 moq-scene on the media clock, so the program, a recording, and a replay match.
-Carrying transitions and keyframes in the layout track, for viewers to
-compose, waits for the m1 [composition epic](/quest/m1/compose/README.md)
-(changed 2026-11-04, maintainer: the program comes first). Overlays animate
-through their own CSS. No Lottie runtime for now (decided 2026-10-26): the
-open ones either lack text (velato) or add a C++ build (ThorVG). Keyframes and
-stinger settings change through moq-scene's `Edit`, so they undo like any edit.
-
-A stinger clip is a media source like any other, but no source plays a media
-file yet, and a stinger must start its clip as the transition starts or its
-cut point lands at random. It waits on a media-file source that plays from
-the start on demand, which no quest plans yet.
-
-Open questions for the maintainer:
-
-- What starts an item's keyframes: its scene going on program (and into
-  preview, for the preview), a loop on the program clock, or a trigger such
-  as a hotkey. Recommended: the time since its scene went on program, playing
-  once and holding the last keyframe.
-- How a keyframed item edits: a drag in the preview sets the keyframe at the
-  preview's time, or the item's resting place, from which keyframes are
-  offsets.
+Carrying keyframes in the layout track, for viewers to compose, waits for the
+m1 [composition epic](/quest/m1/compose/README.md) (changed 2026-11-04,
+maintainer: the program comes first). Overlays animate through their own CSS.
+No Lottie runtime for now (decided 2026-10-26): the open ones either lack text
+(velato) or add a C++ build (ThorVG). Keyframes change through moq-scene's
+`Edit`, so they undo like any edit.
+
+Decided 2026-11-04 (maintainer):
+
+- **Start:** an item's keyframes run on the time since its scene went on
+  program (and into preview, for the preview), play once, and hold the last
+  keyframe. Rejected: a loop on the program clock, and a trigger or hotkey.
+- **Editing:** a drag in the preview moves the item's resting place, and
+  keyframes are offsets from it, so editing needs no timeline scrubber.
+  Rejected: setting the keyframe at the preview's time.
+
+Stingers are their own quest, [Stinger transitions](/quest/m0/stinger.md).
 
 ## Related
 
-- [Layout track](/quest/m1/compose/layout.md) - where transitions and keyframes travel to viewers in m1
+- [Layout track](/quest/m1/compose/layout.md) - where keyframes travel to viewers in m1

quest/m0/media-file.md

@@ -0,0 +1,29 @@
+# [M] Media file source
+
+## Goal
+
+A source in `moq-studio` plays a local video or audio file, such as an intro
+clip or background music: looping, or from the start each time something asks,
+with its sound joining the mixes like any other source.
+
+## Plan
+
+Decided 2026-11-04 (maintainer): planned so stinger transitions have a clip
+that starts on demand. Demux and decode through the moq crates the studio
+already uses (moq-mux import, moq-video and moq-audio decode) rather than a
+new media stack; anything they lack lands in moq first, with a moq-side test,
+then a pin. Start with the container and codecs those crates already read, and
+refuse anything else at setup with the reason. The file path is a local path
+in the scene file, like an image source's. The web studio has no local paths:
+there a file comes from a browser file picker for the session, and resolving
+scene-file paths in the browser belongs to the [web studio](/quest/m0/web.md),
+as it does for image sources.
+
+A file is played on the program clock. Settings: loop, or play once and hold
+the last frame (an audio-only file goes silent after its last sample); and whether going on program restarts it. A restart on demand
+(the stinger's need) is an API on the source, not only a setting. A file that
+ends or fails reads as ended or failed in the Sources panel and Health.
+
+## Related
+
+- [Stinger transitions](/quest/m0/stinger.md) - the first consumer of a clip that starts on demand

quest/m0/stinger.md

@@ -0,0 +1,19 @@
+# [S] Stinger transitions
+
+## Goal
+
+A scene change in `moq-studio` can run a stinger: a video clip played over the
+program that switches scenes at its cut point, as broadcast graphics do.
+
+## Plan
+
+Split from [Keyframes](/quest/m0/animation.md) on 2026-11-04 (maintainer). A
+stinger is a transition kind beside fade, slide, wipe, and move
+([moq-dev/studio#33](https://github.com/moq-dev/studio/pull/33)), drawn on the
+program clock, whose clip is a media file source. It must start its clip as
+the transition starts, or its cut point lands at random. Its settings (clip
+and cut point) change through `Edit`, so they undo.
+
+## Required
+
+- [Media file source](/quest/m0/media-file.md) - a clip that starts on demand