Add scalable OpenSans font test GUI and /kamkeel testfont command

[KAMKEEL]

Motivation

• Provide an in-repo example GUI that demonstrates crisp, scalable text rendering from the existing TTF resource without adding new font assets.
• Give a simple, client-only test entrypoint so maintainers can open the demo in-game and validate renderer behavior across GUI scale settings.

Description

• Added a client GUI kamkeel.npcs.client.gui.GuiFontTest that draws a title, debug info (renderer path, atlas size, GUI scale, scale factor), and sample lines at sizes 8, 12, 16, 24, 32, 48, 64 with baseline guide lines for alignment verification.
• Implemented kamkeel.npcs.client.renderer.font.ScalableFontRenderer which loads assets/customnpcs/OpenSans.ttf from the classpath, builds an SDF atlas and uses an OpenGL shader path for sharp scaling, and falls back to true per-size rebaking (cached atlases) if shader setup fails; texture parameters use linear filtering and CLAMP_TO_EDGE and GL state is preserved around rendering.
• Added a client-only packet GuiFontTestOpenPacket and a new data packet enum value so the GUI can be opened from server-side code, and added kamkeel.npcs.command.TestCommand registered under Kamkeel commands to expose /kamkeel testfont for players.
• Minimal touches to existing registries: packet enum/registration and CommandKamkeel command list updated to wire the packet and command into existing systems.

Testing

• Initialized repository submodules and prepared the workspace, discovered the existing font at src/main/resources/assets/customnpcs/OpenSans.ttf.
• Compiled the project with ./gradlew compileJava and confirmed a successful build: BUILD SUCCESSFUL (no Java compile errors reported).
• No automated runtime/integration tests were added because the change is a client-side GUI feature that requires running the mod in Minecraft to fully validate rendering; compile-time checks passed.

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Codex Task