feat: mousearea, datetime dates, dir(), renpy.image_size and style.rebuild for Ren'Py games

Screens get an invisible mousearea that reports the pointer entering and leaving without taking
clicks. datetime.date/datetime/timedelta support construction and date arithmetic. dir() lists the
game's variables, renpy.style.rebuild() is a no-op, and renpy.image_size answers only for pictures
the browser already holds (otherwise an IOError the game can catch).

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

frontend/tests/engine-behavior.test.mjs

@@ -89,3 +89,23 @@ test("random generators: choices, sample, seeded Random and frozenset", () => {
     assert.ok(renpy.random.randrange(0, 10, 5) % 5 === 0);
   } finally { engine.destroy(); }
 });
+
+test("datetime date arithmetic, dir() and style.rebuild", () => {
+  const { engine } = makeEngine(runtime());
+  try {
+    engine.newStore();
+    const { datetime, renpy, dir } = engine.builtins;
+    const start = datetime.date(2025, 1, 31);
+    const next = start.__binop__("+", datetime.timedelta({ __kw: { days: 1 } }));
+    assert.deepEqual([next.year, next.month, next.day], [2025, 2, 1]);
+    assert.equal(next.weekday(), 5); // Saturday
+    assert.throws(() => datetime.date(2025, 2, 30), /ValueError/);
+    const back = next.__binop__("-", datetime.timedelta({ __kw: { days: 32 } }));
+    assert.deepEqual([back.year, back.month, back.day], [2024, 12, 31]);
+    assert.equal(next.__binop__("-", start).days, 1);
+    engine.store.known_name = 1;
+    assert.ok(dir().includes("known_name"));
+    assert.equal(renpy.style.rebuild(), null);
+    assert.throws(() => renpy.image_size("missing"), /IOError/);
+  } finally { engine.destroy(); }
+});