Keep drags at full speed in large projects (#394)

Dragging a band of a Filter or a Parametric EQ ran at about 30 fps
in a large project. Each frame of a drag announces the changed
project, and every listener of that event ran, also those whose work
nobody sees while the drag goes on.

- FramePreview marks its own frame, read through isDragFrame().
- The Scene view behind another tab skips such a frame and draws on
  coming back, as it already did while a game runs (#265).
- preloadProjectFiles skips it as well. It walks every asset and
  checks every bake, which costs milliseconds in a large project.
- The command that ends a drag emits for itself, and a drag that
  changes nothing ends with a plain preview, so both catch up then.
- This holds for every drag through FramePreview, the knobs of the
  devices in the graph included.

Measured on stein2 with a copy of a project of 162 assets, a band of
a Parametric EQ dragged at 250 pointer moves a second for 3 s:

- Listeners per frame: 17 ms before, 7.4 ms after.
- Hidden Scene view: 814 ms before, 53 ms after.
- preloadProjectFiles: 514 ms before, 0.4 ms after.
- What is left is mostly the Hierarchy and Assets panels on screen
  (about 3 and 1.3 ms a frame). They stay as they are.

Tests:

- Unit frame-preview, play-throttle: 29 passed (1 new)
- Counter-check: without the mark the new test fails
- e2e 34 (melektaus-atx): ALL PASSED, 136 checks
- Gate (melektaus-atx): CHECK EXIT 0, 288 files, 4417 tests

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

src/editor/App.ts

@@ -18,6 +18,7 @@ import { installLoginHook } from './auth/loginHook';
 import { consoleLog } from './console/log';
 import { installDebugHook } from './debugHook';
 import { Editor } from './Editor';
+import { isDragFrame } from './framePreview';
 import { startPlugins } from './plugins/startPlugins';
 import { CelUploadStore } from './pixelart/celUploadStore';
 import { EditorState } from './state';
@@ -74,8 +75,11 @@ export async function startEditor(app: Application, root: VStack): Promise<void>
     // Drawn pictures (#185) are looked up in the project this session holds; the next session
     // installs its own, which is also what lets go of everything the old one had drawn.
     textureLibrary.setPictureFactory(projectPictureFactory(() => state.project));
-    // Asked on every change, which is cheap – a file that is held or on its way is skipped.
-    state.on('project', () => preloadProjectFiles(state.project));
+    // Asked on every change but a drag's frame (#394): in a large project the walk over all assets
+    // costs milliseconds, and the command that ends the drag asks again.
+    state.on('project', () => {
+      if (!isDragFrame()) preloadProjectFiles(state.project);
+    });
     const editor = new Editor(
       app,
       root,