ShaderStages (WebGL2): see vertex and fragment shaders working together

Drag the three vertices, change the image size, and hover a pixel to follow one
fragment from the vertex shader’s outputs to the color the GPU wrote.

Run

npm install
npm run dev      # then open the printed localhost URL
npm run build

Shaders live in ./shaders/*.vert.glsl|*.frag.glsl and are fetched as strings, so there is no inline shader text in the JavaScript.
ES modules need a server; opening app.html from file:// will not work.

Layout

• ../../Common/MV.mjs, ../../Common/initShadersJS.mjs course libraries, unchanged
• ./ShaderDemo.mjs DOM-free renderer (programs via initShaders, VAO, buffers, readback)
• ./main.mjs controller: DOM, pointer events, readouts
• ./overlay.mjs 2D overlay: grid, handles, hovered-pixel weights
• ./math.mjs CPU mirror of vertex transform and rasterizer interpolation (uses MV.mjs)
• ./shaders/ triangle.vert.glsl, color.frag.glsl, barycentric.frag.glsl, fragcoord.frag..glsl

../../Common/ is shared between projects.