Refactor: Uniform optimization + javadoc

[capdevon]

This PR refactors the Uniform class in com.jme3.shader to improve code clarity, maintainability, and usage safety. Key changes include:

• Replacing wildcard and redundant imports with explicit class imports.
• Improving null checks, input validation, and variable naming for clearer intent.
• Unifying and simplifying handling of value assignment in setValue.
• Replacing manual equals implementation with Objects.equals.
• Adding and enhancing Javadoc comments for public methods and class-level documentation.
• Using meaningful constants for location checks and buffer allocation.
• Ensuring proper buffer cleanup in deleteNativeBuffers with explanatory comments.
• General code cleanup, minor bug fixes, and consistency improvements.

These updates make the Uniform class easier to understand, safer to use, and better documented for both users and maintainers.

[github-actions]

🖼️ Screenshot tests have failed.

The purpose of these tests is to ensure that changes introduced in this PR don't break visual features. They are visual unit tests.

📄 Where to find the report:

• Go to the (failed run) > Summary > Artifacts > screenshot-test-report
• Download the zip and open jme3-screenshot-tests/build/reports/ScreenshotDiffReport.html

⚠️ If you didn't expect to change anything visual:
Fix your changes so the screenshot tests pass.

✅ If you did mean to change things:
Review the replacement images in jme3-screenshot-tests/build/changed-images to make sure they really are improvements and then replace and commit the replacement images at jme3-screenshot-tests/src/test/resources.

✨ If you are creating entirely new tests:
Find the new images in jme3-screenshot-tests/build/changed-images and commit the new images at jme3-screenshot-tests/src/test/resources.

Note; it is very important that the committed reference images are created on the build pipeline, locally created images are not reliable. Similarly tests will fail locally but you can look at the report to check they are "visually similar".

See https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-screenshot-tests/README.md for more information

Contact @richardTingle (aka richtea) for guidance if required

[richardTingle]

These screenshot test failures will be related to #2542