Give the three asset tabs one base, and move the viewports into the editor

src/viewport/ held three classes only src/editor/ ever used, and framing.ts sat
under graph/ although the scene view wants the same bounds sum. The graph, the
texture and the container tab repeated the same header, subscription and
disposal code with two silent divergences: the graph tab said "nothing shown"
with null instead of '', and the texture tab refell back to the first asset on
a selection change but not on a project event.

These arrive together because GraphCanvas ties them: it reads framing from its
new home, hands its fold marks to folding.ts, and its '' change is what
GraphEditor must agree with.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/editor/Editor.ts

@@ -3,8 +3,8 @@ import { findAsset, findObjectHolder } from '../core/model/Project';
 import type { ProjectStore } from '../core/store/ProjectStore';
 import type { TextureStore } from '../core/store/TextureStore';
 import { Button, HStack, Splitter, Theme, ui, VStack } from '../ui';
-import { GameViewport } from '../viewport/GameViewport';
-import { SceneViewport } from '../viewport/SceneViewport';
+import { GameViewport } from './viewport/GameViewport';
+import { SceneViewport } from './viewport/SceneViewport';
 import { attachAutosave } from './autosave';
 import { ContainerEditor } from './container/ContainerEditor';
 import { EditorRun } from './EditorRun';
@@ -207,8 +207,6 @@ export class Editor implements PlayHost {
     return this.projects.boot();
   }
 
-  // --- PlayHost --------------------------------------------------------------------------------
-
   showGame(): void {
     this.showTab('game');
   }
@@ -217,8 +215,6 @@ export class Editor implements PlayHost {
     if (this.tab === 'game') this.showTab('scene');
   }
 
-  // --- frame -----------------------------------------------------------------------------------
-
   private tick(dt: number): void {
     this.measureFps(dt);
     this.play.tick(dt);