Compositions: sprites of its own for dropped pictures (#419)

- New section "Sprites of its own": what a drop onto a track makes,
  sharing, deleting, duplicating, pasting and "Make project asset".
- Development: 2048 checks.

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

Compositions.md

@@ -22,6 +22,27 @@ What is stored (`CompositionAsset` in `src/core/model/Asset.ts`): `tracks` (id,
 `fitMedia`) and `clips` (id, track, animation, start in frames). An item whose object is gone stays
 in the list, so an undo finds it again, and plays as nothing.
 
+### Sprites of its own
+
+A picture or a video dragged straight onto a track (#419) needs a sprite to wear it, and a
+composition keeps that sprite to itself, the way a song keeps its takes (see
+[Songs](Songs)): a real sprite asset whose `owner` is the composition. Everything that finds an
+asset by its id finds it, but the asset panel and the pickers leave it out. The inspector names
+it with a dot in front, as the Song tab does with a song's own sounds.
+
+- **One sprite per picture.** A second drop of the same video wears the same sprite. What one item
+  should have differently goes on its object as an override.
+- **It goes with the last item wearing it**, in the same undo step. Deleting the composition takes
+  its sprites along, duplicating it gives the copy sprites of its own.
+- **Pasted into a scene or a container**, the sprite turns into a project asset (named apart from
+  the others), so deleting the composition cannot take the pasted object along. **Pasted into
+  another composition**, that one gets a copy of its own. A cut keeps the sprite in the copy, so a
+  paste after it wears it again.
+- ***Make project asset*** on the item turns the sprite into an ordinary sprite in the asset list.
+
+This is held after every edit (`KeepCompositionSpritesCommand` in
+`src/core/commands/compositionSprites.ts`), so it is taken back and done again with it.
+
 ## The rules
 
 What lives at a frame is one plain function, `compositionAt` (`src/core/model/composition.ts`),
@@ -50,6 +71,10 @@ the clips as narrower bars above them. The header bar has **+ Track**, **Play**
   be placed in a scene can go, a composition excepted: one inside another is refused while it
   hovers. It lands at the frame under the pointer, over an item there too, and is 3 s long, or as
   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.
 - **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
@@ -66,7 +91,8 @@ the clips as narrower bars above them. The header bar has **+ Track**, **Play**
   video, one line per round. An overlap is darkened, and a line marks the end of the item below.
 - **Click** picks an item or a clip, `Ctrl`-click adds or removes one. A click on a track head picks
   the track, a click in the ruler moves the playhead there, and dragging in it scrubs.
-- **The right-click menu** on an item: *Fit to media*, *Remove item*. On a clip: *Open animation*,
+- **The right-click menu** on an item: *Fit to media*, *Make project asset* (only for an item
+  wearing a sprite of the composition's own), *Remove item*. On a clip: *Open animation*,
   *Remove clip*. On a track head or an empty stretch of a track, as in the Song tab: *Add track
   above*, *Add track below*, *Move up*, *Move down*, *Delete track*.
 - **Copy and paste.** `Ctrl+C` on picked items keeps their objects and their timing, `Ctrl+V` in
@@ -136,5 +162,5 @@ with the same row binding the `AnimationSystem` uses.
 ## Not yet
 
 - Sounds and songs as items on a track (#418).
-- Dropping a texture or a video asset straight onto a track (#419); for now a sprite wearing it.
+- Dragging a clip of a video straight out of the Video tab onto a track.
 - Copying clips, a composition inside a composition, and *Sort with parent* for a placement.

Development.md

@@ -21,7 +21,7 @@ php tests/php/releases.test.php                               # the release stor
 php tests/php/files.test.php                                  # uploaded files in a folder per account (#344), the same way
 php tests/php/videos.test.php                                 # the video store, its queue and the worker (#402), with the
                                                               #   real ffmpeg: the CI image and game-maker-video have it
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 2047 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 2048 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result