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/PlaySession.ts

@@ -9,7 +9,7 @@ import { CollisionSystem } from '../runtime/systems/CollisionSystem';
 import { Input } from '../runtime/systems/Input';
 import { World } from '../runtime/World';
 import { ui } from '../ui';
-import type { GameViewport } from '../viewport/GameViewport';
+import type { GameViewport } from './viewport/GameViewport';
 import type { GraphEditor } from './graph/GraphEditor';
 import type { EditorState } from './state';