refactor(wgsl): modularize common shader functions

Extracted common WGSL functions into separate files in `common/shaders/` to improve reusability and maintainability.

- Created `common/shaders/render/fullscreen_vs.wgsl` for a reusable fullscreen vertex shader.
- Created `common/shaders/math/color.wgsl` for color conversion and tone mapping functions.
- Created `common/shaders/math/utils.wgsl` for general math utilities.
- Created `common/shaders/render/raymarching.wgsl` for SDF raymarching logic.
- Updated multiple shaders to use these new common snippets via `#include`.
- Fixed the shader asset validation test to correctly handle shaders that include the common vertex shader.

This refactoring makes the shader code more modular and easier to manage.

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