test: add canvas mock helper for pretext-based engine tests

src/shared/lib/helpers/__mocks__/canvas.ts

@@ -0,0 +1,29 @@
+// src/shared/lib/helpers/__mocks__/canvas.ts
+//
+// Call installCanvasMock(fn) before any pretext import to control measureText.
+// The factory receives the current ctx.font string and the text to measure.
+import { vi } from 'vitest';
+
+export type MeasureFactory = (font: string, text: string) => number;
+
+export function installCanvasMock(factory: MeasureFactory): void {
+    let currentFont = '';
+
+    const mockCtx = {
+        get font() {
+            return currentFont;
+        },
+        set font(f: string) {
+            currentFont = f;
+        },
+        measureText: vi.fn((text: string) => ({ width: factory(currentFont, text) })),
+    };
+
+    // HTMLCanvasElement.prototype.getContext is the entry point pretext uses in DOM environments.
+    // OffscreenCanvas takes priority in pretext; jsdom does not define it so DOM path is used.
+    Object.defineProperty(HTMLCanvasElement.prototype, 'getContext', {
+        configurable: true,
+        writable: true,
+        value: vi.fn(() => mockCtx),
+    });
+}