feat(mod-api): add trainer battle party scope

docs/rfcs/0010-trainer-battle-party-scope.md

@@ -0,0 +1,84 @@
+# RFC 0010: Deferred trainer preparation and battle-local party scope
+
+## Status
+
+Proposed.
+
+## Motivation
+
+Challenge and tournament mods sometimes need a player to choose an eligible
+subset of the save party before a trainer battle. The current public surface
+can replace the opponent through `trainer.party` and observe
+`world.trainer_engaged`, but it cannot pause the engagement before battle
+construction or keep unselected save-party members out of initial send,
+switch, replacement, exhaustion, experience, and party-menu traversal.
+
+Temporarily rewriting `game.save.party` is not a safe substitute: it changes
+authoritative save state, composes poorly with checkpoints and other mods, and
+can strand excluded Pokémon if a callback or process fails.
+
+## Decision and plan extended
+
+This implements **D-AT-001: battle-local Gym registration without save-party
+mutation**, the consuming design decision tracked as capability `AT-SP-001` in
+the Adaptive Trainers implementation plan. The plan file is
+[`docs/superpowers/plans/2026-08-14-adaptive-trainers.md`](https://github.com/MaxTomahawk/gen1recomp-adaptive-trainers/blob/main/docs/superpowers/plans/2026-08-14-adaptive-trainers.md),
+Task 5. The engine delta also extends the additive, guarded public-hook
+decision used by RFC 0007 and the screen facade documented in
+`docs/modding.md`; it deliberately contains none of the consuming mod's Gym
+or party-size policy.
+
+## Exact API delta
+
+Add the guarded hook:
+
+```lua
+mod.hooks:wrap("trainer.before_battle", function(next, game, context, continue)
+  -- context = { trainerClass, partyIndex, mapId, npcId }
+  -- Return true only when the battle has been deferred.
+  -- Call continue() for the full save party, or:
+  -- continue({ playerPartyIndices = { 2, 4, 5 } })
+end)
+```
+
+The hook runs after the trainer's challenge text and immediately before the
+trainer battle is constructed. A mod may push a registered screen with
+`mod.ui.push`, return `true`, and retain `continue` for its confirm/cancel
+callback. `continue` is one-shot and returns `false` after the first call.
+Returning anything other than `true` without calling it continues immediately
+with vanilla scope. With no subscriber, no context or continuation is built.
+
+`playerPartyIndices` is an ordered, one-based list into `game.save.party`.
+Valid unique indices create `battle.playerParty` as a battle-local view of the
+same Pokémon records; the save party itself is never reordered or replaced.
+Malformed or empty scopes degrade to the full party. The view governs initial
+send, all battle party menus and targets, voluntary and forced replacement,
+exhaustion/blackout checks, participant and EXP.ALL traversal, party counts,
+and party-ball presentation. Checkpoints preserve the index list and rebuild
+the same view before restoring battlers.
+
+The API sets no maximum, chooses no members, identifies no boss, and contains
+no scaling or challenge policy.
+
+## Migration and compatibility
+
+Existing mods change nothing. `BattleState.newTrainer(game, class, index)`
+keeps its current behavior; the optional fourth argument is additive. Existing
+battle checkpoints without a party scope restore against the full save party.
+Wild, Safari, link, and no-mod battles are unchanged.
+
+## Verification
+
+- The catalog-driven hook gate proves empty-chain parity and the guarded hot
+  path proves no-mod engagement starts exactly once without allocation.
+- A sandboxed fixture mod defers through its public hook facade, inspects the
+  data-only context, and resumes once with ordered indices.
+- Engine tests cover initial send, party menus, replacement/exhaustion,
+  EXP traversal, invalid-scope fallback, and save-party identity.
+- Battle-checkpoint tests prove scoped capture/restore and old-checkpoint
+  compatibility.
+
+## Deprecation etiquette
+
+Nothing is deprecated. The hook and optional constructor argument are
+additive.