Fix findUsages/undo/node-lookup bugs, split Viewport and Editor, share command and inspector helpers

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

tests/ui/math.test.ts

@@ -0,0 +1,35 @@
+import { describe, expect, it } from 'vitest';
+import { clamp, centreY, zoomFactor } from '../../src/ui/math';
+
+describe('clamp', () => {
+  it('keeps a value inside the range', () => {
+    expect(clamp(5, 0, 10)).toBe(5);
+    expect(clamp(-1, 0, 10)).toBe(0);
+    expect(clamp(11, 0, 10)).toBe(10);
+  });
+
+  it('lets the lower bound win when the range is inverted', () => {
+    // A popup wider than the screen: pinning it to the left edge beats pinning it to the right.
+    expect(clamp(5, 10, 0)).toBe(10);
+  });
+});
+
+describe('centreY', () => {
+  it('centres on whole pixels', () => {
+    expect(centreY(30, 20)).toBe(5);
+    expect(centreY(31, 20)).toBe(6);
+  });
+});
+
+describe('zoomFactor', () => {
+  it('zooms in for a negative delta and out for a positive one', () => {
+    expect(zoomFactor(-100)).toBeGreaterThan(1);
+    expect(zoomFactor(100)).toBeLessThan(1);
+    expect(zoomFactor(0)).toBe(1);
+  });
+
+  it('returns exactly to the starting zoom after scrolling back', () => {
+    // Why exponential: the scene view and the graph editor both rely on this cancelling out.
+    expect(1 * zoomFactor(-120) * zoomFactor(120)).toBeCloseTo(1, 12);
+  });
+});