Texture tab: one masked container for everything the view moves (#82)

Only the picture was masked, so the checkerboard, the frame outlines, the
handles and the name plates were drawn over the panel next door once the
sheet was zoomed past the tab. The dark block in the report is the
checkerboard's base: checker.ts fills the whole image rect and clips only
the light squares to the widget.

Everything that pans and zooms now sits in one masked container, the
pattern WorldViewport and AnimationCanvas already use. A check samples
pixels outside the tab and names the frames drawn past it; a guard in
front of it asserts that zooming really pushes the sheet over every edge,
or the pixel probes would measure nothing and stay green forever.

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

src/editor/debugHook.ts

@@ -315,6 +315,7 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
       pickerWidth: () => textureEditor.picker.w,
       /** Where an image pixel of the shown sheet sits on screen (frames are dragged in image coordinates). */
       pointAt: (x: number, y: number) => textureEditor.imageToScreen(x, y),
+      rect: () => textureEditor.canvasRect(),
     },
     animation: {
       /** Frames, the one on screen, play state and where the preview picture is drawn. */