Clips: rows in the asset panel, drag onto tracks, links (#425)

- Concepts: a sound or video with clips folds out one row per clip in
  the asset panel. A clip row drags like an asset, and a link in the
  inspector can point at a clip itself. The Plays row of an audio item
  names the clip.
- Compositions: a video clip dropped on a track becomes an item as
  long as the clip, wearing its video's sprite with the clip as texture
  override. Sound clips come from the panel too. The "Not yet" line on
  clips of a video is gone.
- Songs: a clip of a sound is dragged from the panel onto a lane.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Compositions.md

@@ -133,12 +133,15 @@ the clips as narrower bars above them. The header bar has **+ Track**, **Play**
   long as the video the sprite wears.
 - **Dragging a texture, vector art, pixel art or a video onto a track** (#419) makes an item the
   same way, wearing a sprite the composition keeps for that picture (see *Sprites of its own*
-  below). A video's item is as long as the video. A clip of a video is chosen afterwards, on the
-  Texture row of the item's inspector.
+  below). A video's item is as long as the video.
+- **Dragging a clip of a video onto a track** (#425) makes an item as long as the clip. It wears
+  the same sprite as its whole video, so a composition keeps one sprite per video, and the clip is
+  the Texture of the item's own object. The clip rows sit under their video in the asset panel
+  (see [Concepts](Concepts)). A clip can also be chosen afterwards, on the Texture row.
 - **Dragging a sound, a clip of one, a pattern or a song onto a track** (#418) makes an audio item
-  there, as long as one pass of it, picked; a song's first pass ends where its loop does. A sound
-  has to be loaded to know its length; the drag loads it, and until it is there the item is 3 s
-  long.
+  there, as long as one pass of it, picked; a song's first pass ends where its loop does. A clip of
+  a sound comes from its row under the sound in the asset panel (#425). A sound has to be loaded
+  to know its length; the drag loads it, and until it is there the item is 3 s long.
 - **Dragging an animation onto a track** makes a clip at that frame, if it fits between the clips
   there.
 - **Dragging an item** moves it in time and from track to track, past and over other items; it
@@ -269,5 +272,4 @@ with the same row binding the `AnimationSystem` uses.
 ## Not yet
 
 - A baked song plays its notes inside a composition, not its bake.
-- Dragging a clip of a video straight out of the Video tab onto a track.
 - A composition inside a composition, and *Sort with parent* for a placement.

Concepts.md

@@ -249,6 +249,11 @@ With **Show empty asset categories and types** in *General settings…* (off by
 panel lists every category with every type, also those without assets. They stand faint, with a
 count of 0, and an empty type has no chevron, since there is nothing to unfold. A right-click on
 such a heading creates the first asset of that type.
+**A sound or a video with clips** has a chevron of its own (#425): it unfolds one row per clip
+under it, folded when the project opens. A clip row is dragged like an asset, onto a track of a
+song or a composition. A click on it shows the clip in the inspector without moving a tab, a
+double-click opens its sound or video with the clip picked. Folding the clips keeps a clip picked
+in its tab.
 A click on a row only
 selects (the inspector shows it, no tab moves); a **double-click** opens the asset as a document
 in a tab of its own (see **The tab row**) — a behavior, state machine, shader, particles,
@@ -280,9 +285,12 @@ and underlined, or gold and underlined while it is an override. A **click pings*
 the way Unity does: the asset panel unfolds its group, scrolls to its row and lets the row light
 up for a second. Nothing is selected, so the inspector stays on the object you are working on.
 A **double-click** opens the asset like a double-click in the asset panel. A frame pings and
-opens the texture or pixel art it was cut from. A row that names nothing is plain text again.
+opens the texture or pixel art it was cut from. A clip of a sound or a video (#425) is a link to
+the clip itself: a click unfolds its sound or video and lights up the clip's row, a double-click
+opens the sound or video with the clip picked. A row that names nothing is plain text again.
 A placed object's **Asset** row (and **Part of** for content of a container) works the same:
-the label is the link, the field beside it shows the asset's name and only reads. It follows a
+the label is the link, the field beside it shows the asset's name and only reads. So does the
+**Plays** row of an audio item in a composition, which links the clip where it plays one. It follows a
 rename of the asset right away.
 
 **Groups in the inspector** (#290). The rows stand in groups, and each group has a **band** as

Songs.md

@@ -81,7 +81,7 @@ right-click menu, not from the inspector.
 
 | Do this | And this happens |
 |---|---|
-| Drag a pattern or a sound from the Assets panel onto a lane | A frame shows where it lands; let go and it lies there, on the grid line before the pointer, as long as the pattern or as the sound at the song's tempo, picked. Left of bar 1 nothing lands. |
+| Drag a pattern, a sound or a clip of one (#425) from the Assets panel onto a lane | A frame shows where it lands; let go and it lies there, on the grid line before the pointer, as long as the pattern or as the sound or clip at the song's tempo, picked. Left of bar 1 nothing lands. |
 | Drag an item by its body | It moves along the grid and to the track under the pointer, the other picked items with it. |
 | Drag an item by its right end | It takes the length to the grid line or the end of a round nearest the pointer, the other picked items lengthen by as much. An end of a round between two ticks is cut to the tick before, so nothing of the next round sounds. |
 | Drag an item by its left end | It is cut back or pulled out at the front, to the grid line or the start of a round nearest the pointer; what it plays stays where it was. Pulled out past the start of what it plays, it repeats it. A sound not loaded yet stops at its start. |