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/config/locale/LocaleLoader.java

@@ -23,7 +23,7 @@ import java.util.stream.Stream;
 /**
  * Loads locale files from disk with validation and auto-fix.
  */
-public class LocaleLoader {
+public final class LocaleLoader {
 
     private LocaleLoader() {
     }