Videos: a cut into the middle of a clip shows no old picture (#404)

- Videos: new point "A cut into the middle of a clip". The view holds
  its last picture while the new clip seeks, at most 0.3 s.
- Development: 2008 checks.

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

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                 # 2007 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 2008 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

Videos.md

@@ -134,6 +134,10 @@ shader's parameters. Or one sprite whose Texture is keyed from clip to clip.
   its picture, not its plain colour
   while the video loads. Each one costs a player while it waits, so a cut with many clips
   holds many videos at once.
+- **A cut into the middle of a clip:** switched on at another picture than the one its player was
+  held at (a scrub, or a Video time that jumps at the cut), the video has to seek first. Until the
+  seek is done the view keeps its last frame, for at most 0.3 s, so the held picture never shows.
+  A seek that takes longer leaves the sprite out until its picture is there.
 - **Play waits for them:** the Game tab opens players of its own at Play. Until each of its driven
   videos has its first picture, the run holds at second 0, for at most 3 s ("Loading the
   videos..."). A released game waits the same way. A driven sprite without its picture is not