Introduce mesh3d_hair GLSL shaders with alpha-blended Vulkan pipeline
(no back-face cull, depth write disabled). Expose gfx.newHairShader(),
Renderable3D.setHair() for a back-to-front transparent pass in
RenderSystem3D, plus HairShader helpers and unit tests.
Co-authored-by: Cursor Agent <cursoragent@cursor.com>