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/human-test-saves.test.ts

@@ -65,5 +65,12 @@ describe.skipIf(!existsSync(romPath) || !process.env.MAKE_TEST_SAVES)(
       expect(world.mapId).toBe("24:4");
       write("fly-teleport", world);
     });
+
+    it("writes headbutt: Route 29 facing a Headbutt tree", () => {
+      // Totodile with Headbutt and Tackle, below the first reachable tree.
+      const world = foundationFixture(assets(), "headbutt");
+      expect(world.mapId).toBe("24:3");
+      write("headbutt", world);
+    });
   },
 );