feat(mod-api): expose field residual descriptors

docs/rfcs/0016-battle-field-residual.md

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+# RFC 0016: Engine-owned field residual descriptors
+
+## Status
+
+Proposed.
+
+## Motivation
+
+Battle-rule mods can keep deterministic data-only state in the public battle
+field and observe `battle.turn_ended`, but that event fires after the engine's
+residual and faint pipeline. A listener cannot safely deal end-of-round field
+damage: directly changing live HP bypasses bar drains, faint messages,
+experience, replacements, double-faint resolution, and checkpoint continuation.
+Putting callbacks into `battle.field` is also rejected by the checkpoint
+serializer, correctly, because executable state is not save-safe.
+
+## Decision and plan extended
+
+This implements **D-AT-004: public engine-owned field residual execution**, the
+consuming decision required by Adaptive Trainers capability `ENGINE-FIELD-
+RESIDUALS`. The plan 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 8. The engine delta defines only a generic end-of-round extension point;
+it contains no weather names, immunities, damage formula, trainer identity, or
+Adaptive Trainers policy.
+
+## Exact API delta
+
+Add the guarded Gen 1 hook:
+
+```lua
+mod.hooks:wrap("battle.field_residual", function(next, context)
+  local rows = next(context)
+  rows[#rows + 1] = {
+    side = "enemy",
+    amount = 7,
+    message = context.battlers.enemy.name .. " is buffeted!",
+  }
+  return rows
+end)
+```
+
+The hook runs once during an undecided battle's end-of-round processing, after
+vanilla status residuals and before field/side token expiry and
+`battle.turn_ended`. With no subscriber, the guarded site builds no context and
+changes nothing. Vanilla contributes an empty list.
+
+`context` is `{ field, battlers, turn }`. `field` is a strictly data-only view
+with the same `{ weather, tokens }` shape captured by battle checkpoints. Its
+recursive projection retains raw tables and finite numbers, strings, and
+booleans under scalar keys. It strips metatables and omits functions, userdata,
+threads, unsupported keys, and cyclic edges. Thus it exposes neither
+`field.sides` nor a graph or executable callback back into live engine state.
+`battlers.player` and `battlers.enemy` are detached snapshots with `{ side,
+name, hp, maxHp, types, vanished }`. Changing either detached view cannot
+change the live battle. `turn` is the current Gen 1 turn counter.
+
+A result row is `{ side = "player"|"enemy", amount =
+positive_finite_integer_number, message = optional_string }`. Numeric strings,
+zero, negatives, fractions, NaN, infinities, malformed sides, and non-string
+messages fail closed. Damage is clamped to current HP. The engine owns
+mutation, HP-bar drain rows, and its existing faint pipeline. It applies every
+accepted row before scheduling newly fainted battlers. If this hook batch
+terminally faints the player with no healthy reserve, it queues only the player
+faint authority, so descriptor order cannot race a blackout against an enemy
+EXP/replacement path. Otherwise it schedules newly fainted battlers in fixed
+player/enemy order. This precedence is scoped to this hook response; native
+faint paths, including `enemyMonFainted`, keep their existing no-hook behavior.
+Wrappers compose by calling `next(context)` and appending their own rows.
+Callbacks are not part of the descriptor contract and hook functions are never
+stored in the battle.
+
+Gold already owns native weather and a generation-specific between-turn order;
+this first additive call site is Gen 1-only. A future Gold site must keep the
+same context and descriptor contract and choose its native ordering explicitly.
+
+## Migration and compatibility
+
+Existing mods change nothing. No hook name or payload changes. With no wrapper,
+Gen 1 performs the same residual, token, event, and faint work as before,
+including native simultaneous-faint resolution, and allocates no context. Gen
+2 is unchanged. Existing and new checkpoints keep
+serializing only the data stored in `battle.field`; hook callbacks remain
+process-local loader state and are never serialized.
+
+The v1 surface remains unchanged: `content.X:register/override/get`,
+`events:on`, `hooks:wrap`, `mod.log`, `mod:read`, manifest v1 fields, and
+`pokemon.before_give` keep their existing behavior.
+
+## Verification
+
+- The catalog hook parity gate proves null and live-empty buses return the
+  vanilla list unchanged.
+- A sandboxed fixture mod exercises the seam through `mod.hooks`, verifies the
+  detached checkpoint-shaped context, applies damage, and reaches the engine
+  faint pipeline.
+- Engine validation tests cover strict number validation (including numeric
+  strings, NaN, infinities, zero, and negatives), nested mutation isolation,
+  omission of functions/userdata/threads/cycles and metatables from the public
+  field projection, optional messages, non-table results, clamping, and
+  settled-battle suppression.
+- Both descriptor orders are driven through queue completion for a simultaneous
+  terminal residual. Each proves player blackout loss with no EXP event,
+  enemy replacement, or replacement UI.
+- A disabled-bus sentinel proves the guard performs no `Runtime.call` or field
+  context construction. An ordering probe proves the enabled hook runs after
+  vanilla status residuals and before token expiry and `battle.turn_ended`.
+- A no-hook native regression proves a simultaneous zero-HP state still enters
+  the pre-existing enemy-faint EXP and win authority outside this hook batch.
+- Capture/restore/capture evidence proves checkpointed field state round-trips
+  while the enabled process-local hook remains installed and callable.
+
+## Docs with the change
+
+`docs/modding.md` documents the Gen 1 timing, detached payload, descriptor
+validation, simultaneous-terminal result, and checkpoint boundary.
+`docs/mod-api-gen2-compat.md` records that Gold does not yet expose the hook.
+No registry or schema changes are involved, so generated registry docs do not
+change.
+
+## Deprecation etiquette
+
+Nothing is deprecated. The hook is additive.