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/battle/constants.ts

@@ -313,6 +313,8 @@ export const C = {
   BATTLETYPE_TRAP: 9,
   BATTLETYPE_FORCEITEM: 10,
   BATTLETYPE_CELEBI: 11,
+  BATTLETYPE_TREE: 8,
+  TREEMON_SLEEP_TURNS: 7,
   BATTLETYPE_SUICUNE: 12,
   WIN: 0,
   LOSE: 1,