feat: enhance entry-point class handling and update documentation

[Statsly-org]

Summary

• Prevents renaming of classes referenced as entrypoints in:
  • plugin.yml (main)
  • paper-plugin.yml (main, bootstrapper, loader)
  • bungee.yml (main)
  • velocity-plugin.json (main)
  • MANIFEST.MF (Main-Class, Premain-Class, Agent-Class)
• Entry-point classes are only excluded if the referenced class exists in the JAR (including inner classes via $).

Implementation notes

• Added EntryPointDiscoverer to extract entrypoint class names from JAR resources/manifest.
• Merged discovered entrypoints with config.excludeClasses() and used the merged list for:
  • ClassMapping
  • MethodMapping
  • FieldMapping
• Added a log line listing preserved entrypoint classes when detected.

Test plan

• Build a sample Paper plugin with plugin.yml and/or paper-plugin.yml containing main/bootstrapper/loader.
• Obfuscate with class renaming enabled.
• Verify the entrypoint class names remain unchanged and the plugin loads on a Paper server.

Files changed

• src/main/java/st3ix/obfuscator/core/EntryPointDiscoverer.java
• src/main/java/st3ix/obfuscator/core/ObfuscationPipeline.java
• src/main/java/st3ix/obfuscator/gui/ObfuscatorGui.java (UI hint text)
• README.md, Features.md (docs)

Closes #47