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

src/main/java/dev/loki/loparkour/duels/SingleDuelsGenerator.java

@@ -33,6 +33,23 @@ public class SingleDuelsGenerator extends ParkourGenerator {
 
     public SingleDuelsGenerator(@NotNull Session session) {
         super(session);
+        forceDefaultBlocks();
+    }
+
+    @Override
+    public void reset(boolean regenerate) {
+        super.reset(regenerate);
+        forceDefaultBlocks();
+    }
+
+    /**
+     * Duels courses use only plain (non-special, non-schematic) blocks so the
+     * player's color is the only material on the course.
+     */
+    private void forceDefaultBlocks() {
+        state.defaultChances.clear();
+        state.defaultChances.put(BlockGenerationType.DEFAULT, 1.0);
+        state.specialChances.clear();
     }
 
     public void setOwnedPlayer(@NotNull ParkourPlayer pp) {
@@ -43,6 +60,11 @@ public class SingleDuelsGenerator extends ParkourGenerator {
         this.blockData = COLORS.get(index % COLORS.size()).createBlockData();
     }
 
+    @Override
+    public @NotNull BlockData selectBlockData() {
+        return blockData == null ? Material.STONE.createBlockData() : blockData;
+    }
+
     @Override
     @NotNull
     public List<ParkourPlayer> getPlayers() {