Refactor RenderManager for improved Type Safety, javadoc, and code clarity

[capdevon]

This PR modernizes the RenderManager class by improving type safety with generics, enhancing code readability, and updating inline documentation for clarity and completeness. Key changes include:

• Replacing raw types with parameterized generics (e.g., RenderPipeline<?>, HashMap<Class<?>, PipelineContext>).
• Simplifying and modernizing for-loops to use enhanced for-each syntax.
• Refactoring viewport calculation variables to local scope, reducing class state.
• Updating and expanding JavaDoc comments for public methods and parameters.
• Improving code style and consistency (e.g., formatting, nullability, and assertion usage).
• Minor bugfixes and code cleanup for better maintainability.

These changes collectively improve code quality, maintainability, and developer experience without altering core functionality.