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>

docs/SESSION_HANDOFF.md

@@ -1,6 +1,6 @@
 # Recovered Claude session
 
-Latest continuation: see **Mom's bank and her purchases: 2026-10-01**
+Latest continuation: see **Headbutt and Rock Smash: 2026-10-02**
 at the end of this file. Earlier sections are historical snapshots.
 
 Recovered on 2026-09-23 from the local Claude Code transcript
@@ -1039,3 +1039,52 @@ Teleport then Fly was checked once in Chromium and WebKit.
 Not made into cards, because they can be automated: emulator comparisons
 of the flight paths and of the Teleport/Dig warps through the reference
 harness.
+
+## Headbutt and Rock Smash: 2026-10-02
+
+Resumed with "continue the plan" on a clean tree after `ee55d34`, taking
+board card #26.
+
+- `src/game/scripts/treemons.ts`: `treeMonEncounter` (`TreeMonEncounter`
+  with `GetTreeMonSet`, `GetTreeMons`, `GetTreeScore` and `GetTreeMon`,
+  including the rare table after the common one), `rockMonEncounter`
+  (`RockMonEncounter`'s 40%) and `treeScore`, all read from the ROM's
+  `TreeMonMaps`, `RockMonMaps` and `TreeMons`.
+- Interpreter: `callasm` runs `TreeMonEncounter` (facing tile in
+  `wPlayerMapX/Y` coordinates, +4), `RockMonEncounter`, `HasRockSmash`
+  (sets `wCurPartyMon`) and `ShakeHeadbuttTree` (a `headbutt` effect after
+  the text). A `tempWild` local stands for `wTempWildMonSpecies`/
+  `wCurPartyLevel`: `randomwildmon` reads it, `readmem wTempWildMonSpecies`
+  answers from it, and the battle clears it with the tree battle type, as
+  `CleanUpBattleRAM` does. The `battle` effect gained `tree`.
+- World: facing a tree runs `AskHeadbuttScript`; HEADBUTT and ROCK SMASH
+  from the party menu queue `HeadbuttFromMenuScript` and
+  `RockSmashFromMenuScript`, the latter with the rock as `hLastTalked`
+  (`MenuFieldMove.lastTalked`). Talking to a rock already ran
+  `SmashRockScript`. The `headbutt` effect lasts 20 ticks
+  (`HEADBUTT_TICKS`) and `world.headbuttTree` gives the renderer the square
+  and frames; objects do not move meanwhile.
+- Battle: `BATTLETYPE_TREE` (`tree` option) prints
+  `PokemonFellFromTreeText` and puts the time of day's `AsleepTreeMons`
+  species to sleep for `TREEMON_SLEEP_TURNS` (`CheckSleepingTreeMon`). The
+  skipped cry is not presented anyway.
+- Renderer: tile $05 over the tree's four tiles for 36 frames, and the
+  `HeadbuttTreeGFX` sprite in `PAL_OW_TREE` with the frameset's timing
+  (three video frames a step, frame 2 mirrored on every other pass). Checked
+  in Chromium with close-up screenshots across the shake.
+- New profile entries (OVERWORLD, BATTLE, `RAM.tempWildMonSpecies`) and
+  constants (`BATTLETYPE_TREE`, `TREEMON_SLEEP_TURNS`, `NUM_TREEMON_SETS`),
+  all verified by `check:reference`.
+
+Checks: 435 unit tests with the local ROM (434 passed, 1 skipped; 135
+passed, 300 skipped without it; new `tests/headbutt-rock-smash.test.ts`,
+and the Rock Smash test in `tests/overworld-rom.test.ts` now smashes the
+rock and meets a Krabby); 123 Chromium/WebKit browser tests passed, 19
+skipped intentionally (a new Headbutt flow with a "headbutt" fixture room
+below a Route 29 tree); native and reference builds; reference, vendor and
+release checks; `git diff --check`. No emulator comparison of the tree
+scores or the shake; no CI or physical-device run. Unreachable in normal
+play so far: no open map has rocks, and Headbutt (the Ilex Forest tutor)
+and Rock Smash (a TM on Route 36) are not obtainable yet.
+
+Next: the remaining field items, MAIL and pack reordering (board #27).