feat(mod-api): expose charge decision hook

docs/rfcs/0011-battle-charge-required.md

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+# RFC 0011: Charge-required battle hook
+
+## Status
+
+Proposed.
+
+## Motivation
+
+A battle-mechanics mod can change damage through `battle.damage` and register
+move effects, but it cannot conditionally skip the first turn of an existing
+charge move. In Gen 1, the engine decides and stores the charge continuation
+before any public effect callback can run. Reaching into `user.charging`,
+`user.chargeReady`, or generation-specific volatile state is private,
+checkpoint-fragile, and would require a mod to duplicate move-pipeline policy.
+
+Weather is the immediate example: a portable sun rule needs Solarbeam to
+resolve on selection while leaving Fly, Dig, PP use, hit resolution, animation,
+and secondary effects to the engine. The capability is generic and useful to
+other ruleset and move-mechanics mods.
+
+## Decision and plan extended
+
+This implements **D-AT-002: charge-stage policy remains mod authority through a
+generic guarded engine decision seam**. The consuming design is tracked in the
+Adaptive Trainers implementation plan,
+[`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 delta follows the additive, guarded hook convention documented by
+Route B in `CONTRIBUTING-mods.md`; it contains no weather, move-id, trainer, or
+Adaptive Trainers policy.
+
+## Exact API delta
+
+Both the Gen 1 and Gen 2 battle engines add this guarded hook:
+
+```lua
+mod.hooks:wrap("battle.charge_required", function(next, ctx)
+  -- ctx = {
+  --   battle = live battle controller,
+  --   user = attacking battler,
+  --   target = defending battler,
+  --   move = merged move record,
+  --   charge = true,
+  --   isCalled = false,
+  -- }
+  if should_resolve_now(ctx) then return false end
+  return next(ctx)
+end)
+```
+
+The call site is the initial-use charge decision, after announcement and PP
+handling but before charge state, invulnerability, charge animation, or charge
+text is created. It runs only when the active engine rules would otherwise
+require a charge. It does not run on the release turn. Returning exactly
+`false` skips that initial charge and continues through the engine-owned move
+pipeline. Any other downstream return preserves the charge. `isCalled` is true
+when Metronome or Mirror Move selected the move.
+
+Gold keeps its native sun decision first, so Solarbeam in native sun already
+requires no charge and does not invoke the hook. Gen 1 link battles use the
+shared Gen 1 move pipeline and therefore receive the same seam; normal link
+mod-compatibility rules continue to govern deterministic peers.
+
+The hot path first calls `Runtime.wantsHook("battle.charge_required")`. With no
+subscriber, no hook payload table is allocated and the existing branch runs
+unchanged.
+
+## Migration and compatibility
+
+Existing mods change nothing. The hook name and payload are additive. With no
+wrapper installed, Red, Blue, Yellow, Gold, and Silver retain their previous
+charge state, PP use, text, animation, accuracy, damage, and native weather
+behavior. Existing charge-move data and effect records require no migration.
+
+A mod adopting the seam should call `next(ctx)` unless it deliberately wants to
+skip this charge. It should not mutate private charge fields or re-run the move.
+
+## Verification
+
+- `tests/engine/battle_charge_required.lua` exercises the real Gen 1 and Gen 2
+  engines through a sandboxed public mod, including false-to-skip, next-to-keep,
+  release-turn behavior, called-move PP semantics, shared payload shape, and
+  native Gold sun behavior.
+- The same test proves no-mod charge/release parity and replaces
+  `Runtime.call` with a sentinel behind a false `Runtime.wantsHook` guard.
+- `tests/engine/gate_hooks.lua` discovers the new catalog name and proves empty
+  chains preserve vanilla values and allocation behavior.
+- `tests/engine/gate_gen2_mod_api.lua` requires a guarded site in both
+  generations and keeps the compatibility reference list complete.
+
+## Deprecation etiquette
+
+Nothing is removed, renamed, superseded, or deprecated.