feat: implement Suika Game (Watermelon Game)

Browser-based physics puzzle game where players drop fruits that merge
into larger fruits on collision, using Matter.js for 2D physics and
Canvas2D for rendering. Includes 11-fruit progression chain, scoring,
game-over detection, mouse/touch input, and Vitest test suite.

src/input.test.js

@@ -0,0 +1,30 @@
+import { describe, it, expect } from 'vitest';
+import { clampX } from './input.js';
+import { FRUITS } from './fruits.js';
+import { CONTAINER_X, CONTAINER_WIDTH } from './constants.js';
+
+describe('input clampX', () => {
+  it('clamps X to stay within container for tier 0 (Cherry)', () => {
+    const radius = FRUITS[0].radius;
+    const minX = CONTAINER_X + radius;
+    const maxX = CONTAINER_X + CONTAINER_WIDTH - radius;
+
+    expect(clampX(0, 0)).toBe(minX);
+    expect(clampX(1000, 0)).toBe(maxX);
+    expect(clampX(250, 0)).toBe(250); // center is fine
+  });
+
+  it('clamps differently for larger fruits', () => {
+    const tier4Radius = FRUITS[4].radius; // Persimmon, radius 32
+    const minX = CONTAINER_X + tier4Radius;
+    const maxX = CONTAINER_X + CONTAINER_WIDTH - tier4Radius;
+
+    expect(clampX(0, 4)).toBe(minX);
+    expect(clampX(1000, 4)).toBe(maxX);
+  });
+
+  it('does not change X when within bounds', () => {
+    const x = CONTAINER_X + CONTAINER_WIDTH / 2;
+    expect(clampX(x, 2)).toBe(x);
+  });
+});