Constant SHADER 

pub const SHADER: &str = r#"// Triangle vertex shader for surfaces and filled plots

struct Uniforms {
    view_proj: mat4x4<f32>,
    model: mat4x4<f32>,
    normal_matrix: mat3x4<f32>,
}

@group(0) @binding(0)
var<uniform> uniforms: Uniforms;

struct VertexInput {
    @location(0) position: vec3<f32>,
    @location(1) color: vec4<f32>,
    @location(2) normal: vec3<f32>,
    @location(3) tex_coords: vec2<f32>,
}

struct VertexOutput {
    @builtin(position) clip_position: vec4<f32>,
    @location(0) color: vec4<f32>,
    @location(1) world_position: vec3<f32>,
    @location(2) normal: vec3<f32>,
    @location(3) tex_coords: vec2<f32>,
}

@vertex
fn vs_main(input: VertexInput) -> VertexOutput {
    var out: VertexOutput;

    let world_position = uniforms.model * vec4<f32>(input.position, 1.0);
    out.clip_position = uniforms.view_proj * world_position;
    out.world_position = world_position.xyz;
    out.color = input.color;
    // normal_matrix is mat3x4 for alignment (see Rust side). In WGSL, mat3x4 * vec3 -> vec4.
    out.normal = normalize((uniforms.normal_matrix * input.normal).xyz);
    out.tex_coords = input.tex_coords;

    return out;
}

@fragment
fn fs_main(input: VertexOutput) -> @location(0) vec4<f32> {
    // Flat colors for 2D plotting (no lighting)
    return input.color;
}
"#;