Use this skill when writing, reviewing, debugging, or porting a Kage shader for Ebitengine — anything involving `//kage:unit pixels`, `ebiten.NewShader`, `DrawTrianglesShader`, `DrawRectShader`, or a `Fragment(dstPos vec4, src0Pos vec2, color vec4) vec4` entry point. Use it especially when translating an existing shader from GLSL, Shadertoy, HLSL, MSL, Godot, or Unity, where the source assumes 0..1 texture coordinates, `gl_FragCoord`, `texture()` sampling, a bottom-left origin, `uniform` declarations, a ternary operator, or straight (non-premultiplied) alpha — every one of which is wrong in Kage. Covers the atlas-relative coordinate model, sampling several source images of different sizes, premultiplied alpha, the Go subset Kage actually accepts, and how to compile-check a shader without a GPU.