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>

tests/editor/graph-framing.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest';
-import { type Box, boxesInBand, frameBoxes } from '../../src/editor/graph/framing';
+import { boundsOf, type Box, boxesInBand, frameBoxes } from '../../src/editor/framing';
 
 const LIMITS = { min: 0.25, max: 2.5 };
 
@@ -83,3 +83,29 @@ describe('fit', () => {
     expect(f).toEqual({ zoom: 1, panX: 100, panY: 50 });
   });
 });
+
+describe('boundsOf', () => {
+  it('encloses every box', () => {
+    const boxes: Box[] = [
+      { id: 'a', x: 10, y: 20, w: 100, h: 60 },
+      { id: 'b', x: -40, y: 5, w: 20, h: 10 },
+    ];
+    expect(boundsOf(boxes)).toEqual({ minX: -40, minY: 5, maxX: 110, maxY: 80 });
+  });
+
+  it('reports nothing for no boxes at all', () => {
+    expect(boundsOf([])).toBeNull();
+  });
+
+  /** The scene view seeds the origin so a template drawn far to one side cannot hide it. */
+  it('keeps a seed inside the result even when every box is far away', () => {
+    const far: Box[] = [{ id: 'a', x: 500, y: 500, w: 10, h: 10 }];
+    const origin = { minX: 0, minY: 0, maxX: 0, maxY: 0 };
+    expect(boundsOf(far, origin)).toEqual({ minX: 0, minY: 0, maxX: 510, maxY: 510 });
+  });
+
+  it('still answers for no boxes when a seed is given', () => {
+    const origin = { minX: 0, minY: 0, maxX: 0, maxY: 0 };
+    expect(boundsOf([], origin)).toEqual(origin);
+  });
+});