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>

src/game/rom/assets.ts

@@ -500,6 +500,10 @@ export class CrystalAssets {
       r.byte(entry + 2),
     );
   }
+  /** HeadbuttTreeGFX: the shaking tree's two 2x2 frames, eight tiles. */
+  headbuttTree(): Uint8Array {
+    return this.reader.slice(OVERWORLD.HeadbuttTreeGFX, 8 * 16);
+  }
   /** GetIcon: a species' two-frame party menu icon (ReadMonMenuIcon and
    * IconPointers), eight tiles. */
   monIcon(species: number): Uint8Array {