feat: duels player colors + elytra sub-modes (M-level)
- Add ParkourGenerator.selectBlockData() hook (forced course material);
BlockSelector checks it first; SingleDuelsGenerator overrides to use the
player's color (16 concrete colors) and forces only DEFAULT block type
- Elytra sub-modes: SpeedDemonGenerator (max-speed score), MinSpeedGenerator
(min-speed check, reset on 6 slow ticks), TimeTrialGenerator (score cap),
CloseGenerator (FLAT rings); + SpeedDemon/MinSpeed/TimeTrial/Close modes,
registration, locales play.elytra.*, config mode-settings.*
- Config.getDouble(path, default) + ConfigAccessor overload
- checkstyle 0/0, PMD 0, build green