Port the remaining battle items, the stats screen and hBattleTurn at the menu

- Run every non-ball item effect usable in battle from the ROM's
  ItemEffects: Full Restore, revives, bitter medicine, Bitter Berry, Ethers
  and Elixers, X items, X Accuracy, Guard Spec, Dire Hit and Poké Doll.
- Read happiness changes from HappinessChanges, including the larger
  level-up gain where a Pokémon was caught.
- Model hBattleTurn at the battle menu, including the trainer AI's writes,
  and drop the status-heal refusal.
- Port the stats screen for battles and Bill's PC.
- Compare the send-out and withdraw texts, RestoreHealth, RestorePP,
  ChangeHappiness and CalcPlayerStats with the original routines.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/NATIVE_PORT.md

@@ -156,7 +156,7 @@ triggers, declining and the repeat offer, and mid-battle reloads. Browser
 tests cover PACK, catching and naming. No emulator comparison of the
 capture routines has been run.
 
-## Switching and healing items in battle
+## Switching, battle items and the stats screen
 
 `engine/battle/core.asm`: `BattleMenu_PKMN` and `TryPlayerSwitch` ("is
 already out", "can't be recalled" while wrapped or blocked by Mean Look,
@@ -181,26 +181,60 @@ switching clears both participant sets as the original does.
 
 `engine/items/item_effects.asm` and `pack.asm`: `BattlePack` shows the Items
 and Balls pockets. `CheckItemContext` gives Oak's "This isn't the time"
-text for items with no battle use. `ItemEffects` is read from the ROM:
-`RestoreHPEffect` (`ItemRestoreHP`, `HealingHPAmounts`, `RestoreHealth`) and
-`StatusHealingEffect` (`UseStatusHealer`, `StatusHealingActions`,
-`HealStatus`, `IsItemUsedOnConfusedMon`) run with `UseItem_SelectMon`, the
-party menu action texts and `UseDisposableItem`. "Won't have any effect"
-and a cancelled menu keep the item and the turn. `wItemEffectSucceeded`
-shares `wBattlePlayerAction`'s byte, so a used item takes the turn.
+text for items with no battle use. `ItemEffects` is read from the ROM and
+dispatches every non-ball effect usable in battle:
+
+- Party items (`.BattleField`): `RestoreHPEffect` (`ItemRestoreHP`,
+  `HealingHPAmounts`, `RestoreHealth`), `StatusHealingEffect`
+  (`UseStatusHealer`, `StatusHealingActions`, `HealStatus`,
+  `IsItemUsedOnConfusedMon`), `FullRestoreEffect`, `ReviveEffect` (half or
+  full HP; a revived participant shares the experience again through
+  `wBattleParticipantsIncludingFainted`), the bitter Energypowder, Energy
+  Root, Heal Powder and Revival Herb (`ChangeHappiness` and "It looks
+  bitter…"), `BitterBerryEffect` and `RestorePPEffect` (Ether, Max Ether,
+  Elixer, Max Elixer, MysteryBerry; `GetMaxPPOfMove`, `BattleRestorePP`, a
+  `move` prompt for `MoveSelectionScreen` whose B reopens the party menu).
+- Battle-only items (`.BattleOnly`): `XItemEffect` (`XItemStats`,
+  `RaiseStat` with `hBattleTurn` on the player, used even when the stat
+  cannot rise, and the X-item happiness gain), X Accuracy, Guard Spec and
+  Dire Hit (set once; a second use keeps the item), and `PokeDollEffect`
+  (ends a wild battle as a draw; Oak's warning in a trainer battle).
+
+"Won't have any effect" and a cancelled menu keep the item and the turn.
+`wItemEffectSucceeded` shares `wBattlePlayerAction`'s byte, so a used item
+takes the turn. `UseItemText` waits for a button. `ChangeHappiness` reads
+`HappinessChanges` and updates the battle Pokémon's copy only when
+`wCurPartyMon` equals the 1-based `wPartyMenuCursor`, as in the original;
+fainting and levelling up use it too, and `LevelUpHappinessMod` gives the
+larger gain on the map where the Pokémon was caught.
+
 `CalcPlayerStats` reapplies the paralysis and burn penalties to the side
 named by `hBattleTurn` after a switch of turns, which can be the enemy.
-Because `hBattleTurn` at the battle menu is not modelled everywhere, a
-status heal that would reapply a penalty to a paralysed or burned enemy is
-refused. Other item effects (X items, Poké Doll, Full Restore, revives,
-Ether) are refused.
+`hBattleTurn` at the battle menu is modelled: `DoBattle` leaves the
+player's turn, `HandleHealingItems` ends a full turn on the enemy's, and
+the trainer AI reports when `AI_Types`, `AI_Status` or `AIDamageCalc`
+set it. A status heal after a full turn therefore penalises a paralysed
+or burned enemy again, as the original does. Remaining gaps: the special
+Kurt and Park balls.
+
+`engine/pokemon/stats_screen.asm`: the stats screen's three pages (HP,
+status, types, experience to the next level; item and moves with PP; ID,
+OT and stats) with the ROM's strings, `TypeNames`, `GetGender` and the
+front picture. It opens from BattleMonMenu (B returns to the party menu,
+as `BattleMenuPKMN_ReturnFromStats` does) and from Bill's PC, where a
+boxed Pokémon shows `CalcTempmonStats`' full HP and no status.
 
 Tests: local-ROM battles cover switching, cancelling, the withdrawal texts,
 forced choices (including a fainted pick and a cancelled forced menu), run
 after NO, Shift mode's offer (accepted and declined), a trainer double
-faint, Potion, Max Potion, a Berry on a benched Pokémon, no-effect cases,
-Antidote and PSNCUREBERRY, Parlyz Heal restoring Speed, Oak's warning and a
-refused X Attack. Browser tests switch through the party menu and its
-SWITCH/STATS/CANCEL submenu and use a Potion from the Items pocket. The
-stats screen is not ported. No emulator comparison of these routines has
-been run.
+faint, and every item effect above, including the reapplied enemy penalty
+and the caught-location level-up bonus. Trainer AI tests pin its
+`hBattleTurn` reporting. The reference suite runs the original
+`SendOutMonText` (its final jump to `BattleTextbox` patched to `ret` in the
+test's ROM copy), `WithdrawMonText`'s text routine, `RestoreHealth`,
+`RestorePP`, `ChangeHappiness` and `CalcPlayerStats` with both values of
+`hBattleTurn`: 300, 300, 200, 300 and 200 cases. Browser tests switch
+through the party menu, use a Potion from the Items pocket and page
+through the stats screen. `WithdrawMonText` still assumes the leftover
+multiplicand high byte is zero; a whole battle has not been compared with
+the original.