Split the debug hook into one file per topic (#215)

debugHook.ts had grown to 1097 lines of unrelated test hooks. They
now live in seven files under src/editor/debug/, grouped by topic.
debugHook.ts only assembles them and is down to 86 lines.

- The shape of window.__gm is unchanged for tools/. Measured in the
  browser before and after: 120 top-level keys and 313 leaves with
  their arity, byte-identical JSON.
- The Text import is explicit in the helpers. Without it the name
  fell back to the DOM type and instanceof would never have matched.

Findings:
- The comment for vectorArt sat above pixelArt. It is back in place.
- An orphaned duplicate comment on assetRow is gone.

Tests:
- ./check.sh: 145 files, 2192 tests, CHECK EXIT 0 (unchanged)
- e2e 1-16,37,39-43,47,48: ALL PASSED, 734 checks
- The shape probe turns red against a deliberately misnested hook.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/debug/hookHelpers.ts

@@ -0,0 +1,35 @@
+import type { Application } from 'pixi.js';
+import type { Editor } from '../Editor';
+import type { SnapToggle } from '../graph/SnapToggle';
+import type { EditorState } from '../state';
+
+/** What every group of test hooks is built from. */
+export interface HookContext {
+  app: Application;
+  state: EditorState;
+  editor: Editor;
+}
+
+export function centreOf(widget: { getGlobalPosition(): { x: number; y: number }; w: number; h: number }): {
+  x: number;
+  y: number;
+} {
+  const p = widget.getGlobalPosition();
+  return { x: p.x + widget.w / 2, y: p.y + widget.h / 2 };
+}
+
+/** A tab's magnet (#194): where to click it, how big it is drawn, and whether it is on. */
+export function snapButton(snap: SnapToggle): {
+  x: number;
+  y: number;
+  w: number;
+  h: number;
+  toggled: boolean;
+} {
+  return {
+    ...centreOf(snap.button),
+    w: snap.button.w,
+    h: snap.button.h,
+    toggled: snap.button.toggled,
+  };
+}