Open Route 30 with map trainers and multi-Pokémon trainer battles

Map trainers run the ROM's TalkToTrainerScript and SeenByTrainerScript.
_CheckTrainerBattle runs before tile events at the end of each step and
on idle ticks; TrainerWalkToPlayer builds wMovementBuffer natively, and
writeobjectxy keeps the trainer where it stopped. The interpreter gains
the trainer header commands, wRunningTrainerBattleScript and hidden
items (HiddenItemScript).

Trainers with several Pokémon send out the next one through
EnemyPartyMonEntrance and FindMonInOTPartyToSwitchIntoBattle, including
its BattleRandom fallback. AI_SwitchOrTryItem, CheckAbleToSwitch and
AI_Switch are translated with their original bugs, and wPlayerUsedMoves
is recorded. The Shift-mode switch offer, trainer items, Pursuit on a
switch, Perish Song and Spikes remain refused.

Route 30 and its Berry house are enabled; Mr. Pokémon's house and
Route 31 stay closed. A refused trainer sighting does not restart until
the player moves, so a rollback cannot trap the player in view.

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

docs/NATIVE_PORT.md

@@ -73,8 +73,18 @@ Native saves are version 3. Version 2 added scene values by owning map, phone co
 
 `game/battle/ai.ts` translates `AIChooseMove`, `AI_Redundant` and the Basic, Setup, Types, Offensive, Opportunist, Aggressive, Cautious, Status and Risky layers. It preserves the original random tie selection, known redundancy quirks, HP thresholds, exclusions and the deterministic pre-variation damage estimate used by Aggressive and Risky. Smart remains refused, including the rival's AI. Synthetic scoring vectors establish the tested branches; original-engine AI execution parity is still pending.
 
-`loadtrainer`, `winlosstext` and `startbattle` now run single-Pokémon trainer battles through the existing battle UI. Tests cover Joey's ROM party/text/reward, RUN refusal, trainer experience, Amulet Coin, CANLOSE behavior, money limits, transactional rollback and midbattle restore. Chromium and WebKit exercise the UI with a test-only script. Production map gates are unchanged. Trainer items, party switching, sight lines, trainer-object interaction and the rival round trip remain unported.
+`loadtrainer`, `winlosstext` and `startbattle` run scripted trainer battles through the existing battle UI. Tests cover Joey's ROM party/text/reward, RUN refusal, trainer experience, Amulet Coin, CANLOSE behavior, money limits, transactional rollback and midbattle restore. Chromium and WebKit exercise the UI with a test-only script.
+
+## Map trainers
+
+`home/trainers.asm` and `engine/events/trainer_scripts.asm`: talking to an `OBJECTTYPE_TRAINER` object runs `TalkToTrainerScript` with the object's header copied as `wTempTrainer`; `_CheckTrainerBattle` runs `SeenByTrainerScript` with `wSeenTrainerDistance`. The world checks sight in `PlayerEvents` order: before warps, coordinate events and encounters at the end of a step (a seen player's tile events are then skipped, as `DisableEvents` does), and on idle ticks before scene scripts and input. Walls and objects do not block sight. `TrainerWalkToPlayer`/`ComputePathToWalkToPlayer` build the raw `wMovementBuffer` bytes, which `applymovementlasttalked` decodes with the normal movement reader. `wRunningTrainerBattleScript` lives in the script run: zero at the start, set by `loadmem` after the battle so `endifjustbattled` ends the after-script. Trainer commands without a map trainer are refused. `MomTriesToBuySomething` after scripted battles does nothing while Mom's savings (not ported) are zero.
+
+`engine/battle/core.asm` and `engine/battle/ai/items.asm`, `switch.asm`: when a trainer's Pokémon faints and others remain, `UpdateBattleStateAndExperienceAfterEnemyFaint` zeroes its party HP and runs `NewEnemyMonStatus`; `EnemyPartyMonEntrance` then sends out the Pokémon chosen by `FindMonInOTPartyToSwitchIntoBattle`. When every candidate is threatened it draws `BattleRandom & 7`, which tests pin. `AI_SwitchOrTryItem` runs before either side moves: `CheckAbleToSwitch`, the Often/Rarely/Sometimes thresholds, `AI_TrySwitch` and `AI_Switch` (withdraw text, HP and status back to the party). `CheckTypeMatchup` ignores the type its switch-AI callers load and uses `BATTLE_VARS_MOVE_TYPE`, so every such matchup uses the enemy's chosen move type; `FindEnemyMonsWithASuperEffectiveMove` never resets its result between Pokémon; `FindEnemyMonsWithAtLeastQuarterMaxHP` rotates HP bits instead of multiplying. All three bugs are kept. `UsedMoveText` records `wPlayerUsedMoves`. Refused: the Shift-mode offer to switch (`OfferSwitch`), `AskUseNextPokemon` with other Pokémon left, Pursuit on a switching Pokémon, Perish Song, Spikes, and trainer items. Side effects of the type scans on the move structs (overwritten types and animation) are not modelled; the next `ParseEnemyAction`/`ParsePlayerAction` reloads them.
+
+`BGEVENT_ITEM` runs `HiddenItemScript` with `wHiddenItemData`; `SetMemEvent` sets the item's flag.
+
+A trainer sighting whose script is refused is not restarted until the player moves. This is a native safeguard, not original behavior.
 
 ## Next slice
 
-Route 29 and Cherrygrove are enabled with item balls, fruit trees and the daily reset, the Pokémon Center nurse and the recorded whiteout spawn. NPCs roam. Next: trainer sight lines and trainer battles, capture with the catching tutorial, and party, bag and Pokégear menus over the native models. Other specials stay refused until translated.
+Route 29, Cherrygrove and Route 30 are enabled with item balls, hidden items, fruit trees and the daily reset, the Pokémon Center nurse, the recorded whiteout spawn and map trainers. NPCs roam. Next: Mr. Pokémon's house and the Mystery Egg, capture with the catching tutorial, Reflect/Light Screen, and party, bag and Pokégear menus over the native models. Other specials stay refused until translated.