build: two-tier checkstyle gate, empty suppressions, fix baseline

Checkstyle is now a ratchet: severity="error" only for rules that are at
zero violations, design-debt rules report as warnings and must be flipped
to error as soon as their backlog hits zero (tracked in TODO.md).

- thresholds tightened: MethodLength 80->40, CyclomaticComplexity 20->15,
  NestedIfDepth 3->2, ParameterNumber 7->5; new rules ReturnCount(5),
  FinalClass, HiddenField, VisibilityModifier; MagicNumber and
  FinalLocalVariable promoted from warning to error (both clean)
- suppressions.xml emptied: 45 legacy entries removed, never add back
- fixed 102 baseline violations (431->329): FinalClass in 37 classes,
  WhitespaceAround in 11 files
- TODO.md: Task 1 done (lolib removed, smoke test passed) and Task 2
  baseline recorded (red report, per-package list, next steps)

src/main/java/dev/loki/loparkour/menu/core/MenuStub.java

@@ -4,5 +4,5 @@ package dev.loki.loparkour.menu.core;
  * Placeholder during menu migration; real flows use {@link Menus} / InventoryGui.
  */
 public final class MenuStub {
-    private MenuStub() {}
+    private MenuStub() { }
 }

src/main/java/dev/loki/loparkour/menu/core/screen/DynamicMenu.java

@@ -84,5 +84,5 @@ public abstract class DynamicMenu {
             int id,
             BiFunction<@NotNull Player, @Nullable ParkourUser, dev.loki.loparkour.menu.gui.MenuItem> item,
             Predicate<Player> predicate
-    ) {}
+    ) { }
 }