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/framePreview.ts

@@ -2,6 +2,15 @@ import { cancelFrameWork, onFrame } from '../ui/render';
 import type { EditorState } from './state';
 
 let instances = 0;
+let inDragFrame = false;
+
+/**
+ * Whether the `project` event going on is a drag's frame. What nobody sees during it – a hidden view,
+ * files to load ahead – may wait for the command that ends the drag (#394).
+ */
+export function isDragFrame(): boolean {
+  return inDragFrame;
+}
 
 /**
  * A drag's `state.preview()`, at most once per frame (#213). The gesture still writes the model on
@@ -20,7 +29,14 @@ export class FramePreview {
   }
 
   request(): void {
-    onFrame(this.key, () => this.state.preview());
+    onFrame(this.key, () => {
+      inDragFrame = true;
+      try {
+        this.state.preview();
+      } finally {
+        inDragFrame = false;
+      }
+    });
   }
 
   cancel(): void {