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.js

@@ -0,0 +1,44 @@
+import { FRUITS } from './fruits.js';
+import { CONTAINER_X, CONTAINER_WIDTH } from './constants.js';
+
+export function clampX(x, fruitTier) {
+  const radius = FRUITS[fruitTier].radius;
+  const minX = CONTAINER_X + radius;
+  const maxX = CONTAINER_X + CONTAINER_WIDTH - radius;
+  return Math.max(minX, Math.min(maxX, x));
+}
+
+export function setupInput(canvas, game) {
+  function getCanvasX(clientX) {
+    const rect = canvas.getBoundingClientRect();
+    const scaleX = canvas.width / rect.width;
+    return (clientX - rect.left) * scaleX;
+  }
+
+  canvas.addEventListener('mousemove', (e) => {
+    game.setCursorX(getCanvasX(e.clientX));
+  });
+
+  canvas.addEventListener('click', (e) => {
+    if (game.state.isGameOver) {
+      game.restart();
+    } else {
+      game.drop();
+    }
+  });
+
+  canvas.addEventListener('touchmove', (e) => {
+    e.preventDefault();
+    const touch = e.touches[0];
+    game.setCursorX(getCanvasX(touch.clientX));
+  }, { passive: false });
+
+  canvas.addEventListener('touchend', (e) => {
+    e.preventDefault();
+    if (game.state.isGameOver) {
+      game.restart();
+    } else {
+      game.drop();
+    }
+  }, { passive: false });
+}