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/foundation-browser.spec.ts

@@ -18,7 +18,8 @@ async function start(
     | "candy"
     | "pokegear"
     | "mom"
-    | "fly" = false,
+    | "fly"
+    | "headbutt" = false,
 ) {
   await page.route("**/src/game/runtime.ts", async (route) => {
     const response = await route.fetch(),
@@ -480,6 +481,32 @@ test("Fly picks a visited town on the Fly map and lands there", async ({
   expect([state.map, state.x, state.y]).toEqual(["26:3", 29, 4]);
 });
 
+test("Headbutt shakes a tree and a sleeping Hoothoot falls out", async ({
+  page,
+}) => {
+  await start(page, "headbutt");
+  const text = page.locator("#dialogue-text"),
+    choices = page.locator("#dialogue-choice"),
+    screen = page.locator("#game-screen");
+  await page.keyboard.press("KeyX");
+  await expect(text).toHaveText(/A POKéMON could be\s+in this tree\./);
+  await page.keyboard.press("KeyX");
+  await expect(text).toHaveText(/Want to HEADBUTT\s+it\?/);
+  await choices.getByRole("button", { name: "YES" }).click();
+  await expect(text).toHaveText(/TOTODILE did a\s+HEADBUTT!/);
+  await page.keyboard.press("KeyX");
+  await expect(page.locator("#native-dialogue")).toBeHidden();
+  await page.clock.runFor(5_000);
+  await expect(page.locator("#battle")).toBeVisible();
+  await expect(page.locator("#battle-text")).toHaveText(
+    /HOOTHOOT fell\s+out of the tree!/,
+  );
+  await expect(page.locator("#battle-foe-name")).toHaveText("HOOTHOOT");
+  await expect(page.locator("#battle-foe-status")).toHaveText("SLP");
+  await expect(page.locator("#game-notice")).toHaveText("");
+  await expect(screen).toHaveAttribute("data-map", "24:3");
+});
+
 test("Mom keeps the player's money with the digit box", async ({ page }) => {
   await start(page, "mom");
   const text = page.locator("#dialogue-text"),