docs: correct @builtin(position) explanation per WGSL spec

docs/01-rainbow-triangle.md

@@ -572,8 +572,22 @@ and delivers the interpolated value to every fragment.
 **`@fragment fn fs_main(input: VertexOutput)`** — `@fragment` declares the
 fragment shader entry point. `input` is the rasterizer's interpolated output
 from the vertex shader. Every `@location(n)` field in `VertexOutput` is now
-pre-blended. The `@builtin(position)` field is not interpolated — it is the
-original vertex position.
+pre-blended with barycentric weights.
+
+> **Key insight — TWO `@builtin(position)` builtins, zero connection:**
+> Vertex `@builtin(position)` and fragment `@builtin(position)` are two
+> completely separate builtins that happen to share the same name. The vertex
+> shader outputs clip-space coordinates into `@builtin(position)` for the
+> rasterizer to perform perspective division and viewport transform. The
+> fragment shader receives an entirely different `@builtin(position)` injected
+> by the fragment stage, providing framebuffer pixel coordinates: `x`/`y` are
+> the pixel center within the viewport, `z` is the depth value (typically
+> [0, 1]), and `w` is the interpolated reciprocal of the vertex clip-space
+> w-coordinate (1/w). The vertex shader's position output is NOT passed to the
+> fragment shader's position input. They are independent builtins from
+> different pipeline stages. If you need to pass data from vertex to fragment
+> with interpolation, use `@location(N)` on regular struct fields — which is
+> exactly what `vertex_color` does in our shader.
 
 **`-> @location(0) vec4<f32>`** — The fragment shader must output at least one
 color value at `@location(0)`. This number must match the corresponding color