Fix battery capacity reverting to default on world reload

[Copilot]

Battery capacity (set via crafting multiple batteries together) was silently discarded on save/load — SimpleEnergyHandler.serialize() only persists energy, not capacity. On reload, the block entity's energy handler was re-constructed with the default 1M capacity, capping stored energy even when the actual capacity was higher.

Changes

• SimpleEnergyHandlerCapacity: Override serialize/deserialize to also persist the capacity field. Deserialize falls back to the current (constructor-provided) value when the field is absent, preserving backward compatibility with existing saves.

@Override
public void serialize(ValueOutput output) {
    super.serialize(output);
    output.putInt("capacity", capacity);
}

@Override
public void deserialize(ValueInput input) {
    super.deserialize(input);
    capacity = Math.max(0, input.getIntOr("capacity", capacity));
}

• GameTestsBattery: Add testBatteryCapacityPersistence — sets a 2× capacity handler with energy above the default cap, round-trips through serialize/deserialize, asserts both capacity and energy are preserved.

[coveralls]

coverage: 53.974% (+0.03%) from 53.947% — copilot/fix-battery-revert-bug into master-26