Port Headbutt and Rock Smash encounters

Facing a Headbutt tree asks to HEADBUTT it, and HEADBUTT from the party
menu skips the question. ShakeHeadbuttTree shakes the tree as a sprite over
grass, then TreeMonEncounter scores the facing tile against the trainer ID
and draws from the map's common or rare tree table. A Pokémon that falls
fights as BATTLETYPE_TREE: "fell out of the tree!", and the time of day's
AsleepTreeMons species start asleep.

Smashable rocks run SmashRockScript, and ROCK SMASH from the party menu
acts on the facing rock; after the rock breaks, RockMonEncounter gives the
map's rock set a 40% chance of a wild battle.

Closes #26

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

tests/overworld-rom.test.ts

@@ -537,7 +537,7 @@ describe.skipIf(!existsSync(romPath))("field moves with the local ROM", () => {
     closeMenus(world);
   });
 
-  it("recognises a smashable rock for Rock Smash, which is not ported further", () => {
+  it("recognises a smashable rock for Rock Smash and smashes it", () => {
     // A rock (SPRITEMOVEDATA_SMASHABLE_ROCK) in Dark Cave at 27, 6.
     const spot = { map: "3:78", x: 27, y: 6 };
     const { assets, world } = fixture([spot.map]);
@@ -552,10 +552,19 @@ describe.skipIf(!existsSync(romPath))("field moves with the local ROM", () => {
     expect(useFromMenu(world, "ROCK SMASH")).toEqual(["Can't use that\nhere."]);
     closeMenus(world);
     world.restore({ ...world.save(), direction: "up" });
-    useFromMenu(world, "ROCK SMASH");
-    expect(world.notice).toBe("Smashing rocks is not ported yet.");
-    closeMenus(world);
-    expect(world.diagnostic).toBe("");
+    const shown = useFromMenu(world, "ROCK SMASH");
+    for (let i = 0; i < 10 && world.dialogue?.choice; i++) world.advanceDialogue();
+    // Random always returns 0 here, so the rock hides a Krabby.
+    for (let tick = 0; tick < 500 && !world.battle; tick++) {
+      if (world.dialogue) {
+        shown.push(world.dialogue.pages[world.dialogue.index]);
+        world.advanceDialogue();
+      } else world.tick();
+    }
+    expect(world.notice, world.diagnostic).toBe("");
+    expect(shown).toContain("SWIMMY used\nROCK SMASH!");
+    expect(world.objects().some((o) => o.x === spot.x && o.y === spot.y)).toBe(false);
+    expect(world.battle?.engine.enemy.mon.species).toBe(98); // KRABBY
   });
 
   it("draws a wild Pokémon with Sweet Scent in grass, and nothing elsewhere", () => {