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/rendering/renderer.ts

@@ -10,6 +10,11 @@ import {
   type RenderViewport,
 } from "./viewport";
 
+/** Video frames before LoadOverworldTilemapAndAttrmapPals puts the
+ * Headbutt tree back: HideHeadbuttTree's WaitBGMap and the 32 shaking
+ * frames. */
+const HEADBUTT_HIDDEN_FRAMES = 36;
+
 export class WorldRenderer {
   width: number = BASE_VIEWPORT.width;
   height: number = BASE_VIEWPORT.height;
@@ -91,6 +96,10 @@ export class WorldRenderer {
     const cameraX = Math.round(player.x - (this.width / 2 - 8)),
       cameraY = Math.round(player.y - (this.height / 2 - 8)) + world.screenShake;
     this.image.data.fill(255);
+    // HideHeadbuttTree puts tile $05 (grass) in the tree's four tilemap
+    // places, keeping their attributes, until the shaking ends.
+    const tree = world.headbuttTree;
+    const hidden = tree && tree.frames < HEADBUTT_HIDDEN_FRAMES ? tree : undefined;
     for (
       let tileY = Math.floor(cameraY / 8);
       tileY < Math.ceil((cameraY + this.height) / 8);
@@ -111,7 +120,12 @@ export class WorldRenderer {
         const encoded = tileset.metatiles[block * 16 + localY * 4 + localX];
         const packed = tileset.paletteMap[encoded >> 1],
           attribute = encoded & 1 ? packed >> 4 : packed & 15;
-        const tile = (encoded & 0x7f) + (attribute & 8 ? 0x60 : 0);
+        const covered =
+          hidden &&
+          tileX >> 1 === hidden.x &&
+          tileY >> 1 === hidden.y;
+        const tile =
+          (covered ? 0x05 : encoded & 0x7f) + (attribute & 8 ? 0x60 : 0);
         drawTile(
           this.image.data,
           this.width,
@@ -200,6 +214,31 @@ export class WorldRenderer {
         );
       }
     }
+    if (tree) {
+      // SPRITE_ANIM_OBJ_HEADBUTT over the tree's square, in PAL_OW_TREE:
+      // the two frames alternate every three animation steps, the second
+      // mirrored on every other pass. The first step lasts through
+      // HideHeadbuttTree's WaitBGMap, the last until the sprite is cleared.
+      const graphics = this.assets.headbuttTree(),
+        step = Math.min(32, Math.max(0, tree.frames - 4)),
+        phase = Math.floor(step / 3) % 4,
+        flip = phase === 3,
+        palette = this.assets.objectPalette(6, time);
+      for (let tile = 0; tile < 4; tile++) {
+        const column = tile & 1;
+        drawTile(
+          this.image.data,
+          this.width,
+          this.height,
+          graphics,
+          (phase & 1) * 4 + tile,
+          tree.x * 16 - cameraX + (flip ? 8 - column * 8 : column * 8),
+          tree.y * 16 - cameraY + (tile >> 1) * 8,
+          palette,
+          { transparent: true, flipX: flip },
+        );
+      }
+    }
     this.context.putImageData(this.image, 0, 0);
   }
   private actor(