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>

tests/editor/frame-preview.test.ts

@@ -19,7 +19,7 @@ import type {
 import { PALETTE_KEY } from '../../src/core/model/pixelart';
 import { createEmptyProject } from '../../src/core/model/Project';
 import { CurveCanvas } from '../../src/editor/curve/CurveCanvas';
-import { FramePreview } from '../../src/editor/framePreview';
+import { FramePreview, isDragFrame } from '../../src/editor/framePreview';
 import { GraphDrags } from '../../src/editor/graph/graphDrags';
 import type { GraphCanvasHost } from '../../src/editor/graph/graphHost';
 import type { InspectorHost } from '../../src/editor/inspector/host';
@@ -89,6 +89,18 @@ describe('FramePreview', () => {
     expect(state.preview).not.toHaveBeenCalled();
   });
 
+  it('says during its own preview that the event is a drag frame, and at no other time (#394)', () => {
+    const tick = frames();
+    const seen: boolean[] = [];
+    const state = { preview: vi.fn(() => seen.push(isDragFrame())) };
+    new FramePreview(state, 'drag').request();
+    expect(isDragFrame()).toBe(false);
+    tick();
+    state.preview();
+    expect(seen).toEqual([true, false]);
+    expect(isDragFrame()).toBe(false);
+  });
+
   it('keeps two gestures of the same name apart', () => {
     const tick = frames();
     const a = fakeState();