Constant USER_WARP_FRAGMENT_PREFIX 

const USER_WARP_FRAGMENT_PREFIX: &str = r#"
@fragment
fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
    uv = in.uv;
    // `uv_orig` is the *pre-warp* screen-space UV. Warp HLSL authors
    // sometimes reference it to compare the displaced sample against
    // the pixel's own coordinate (radial fades, vignette terms).
    uv_orig = in.uv_screen;
    rad = length(in.uv_screen - vec2<f32>(0.5)) * 1.4142135;
    ang = atan2(in.uv_screen.y - 0.5, in.uv_screen.x - 0.5);
    ret = textureSample(sampler_main_texture, sampler_main, uv).rgb;
    _md2_color = ret;

    texsize = uniforms.texsize;
    aspect = uniforms.aspect;
    time = uniforms.time;
    fps = uniforms.fps;
    frame = uniforms.frame;
    progress = uniforms.progress;
    bass = uniforms.bass;
    mid = uniforms.mid;
    treb = uniforms.treb;
    vol = uniforms.vol;
    bass_att = uniforms.bass_att;
    mid_att = uniforms.mid_att;
    treb_att = uniforms.treb_att;
    vol_att = uniforms.vol_att;
    rand_preset = uniforms.rand_preset;
    rand_frame = uniforms.rand_frame;
    slow_roam_cos = uniforms.slow_roam_cos;
    slow_roam_sin = uniforms.slow_roam_sin;
    roam_cos = uniforms.roam_cos;
    roam_sin = uniforms.roam_sin;
    blur1_min = uniforms.blur1_min;
    blur1_max = uniforms.blur1_max;
    blur2_min = uniforms.blur2_min;
    blur2_max = uniforms.blur2_max;
    blur3_min = uniforms.blur3_min;
    blur3_max = uniforms.blur3_max;
    hue_shader = vec3<f32>(1.0, 1.0, 1.0);
    g_fTexSize = uniforms.texsize;

    q1 = uniforms.q[0].x;
    q2 = uniforms.q[0].y;
    q3 = uniforms.q[0].z;
    q4 = uniforms.q[0].w;
    q5 = uniforms.q[1].x;
    q6 = uniforms.q[1].y;
    q7 = uniforms.q[1].z;
    q8 = uniforms.q[1].w;
    q9 = uniforms.q[2].x;
    q10 = uniforms.q[2].y;
    q11 = uniforms.q[2].z;
    q12 = uniforms.q[2].w;
    q13 = uniforms.q[3].x;
    q14 = uniforms.q[3].y;
    q15 = uniforms.q[3].z;
    q16 = uniforms.q[3].w;
    q17 = uniforms.q[4].x;
    q18 = uniforms.q[4].y;
    q19 = uniforms.q[4].z;
    q20 = uniforms.q[4].w;
    q21 = uniforms.q[5].x;
    q22 = uniforms.q[5].y;
    q23 = uniforms.q[5].z;
    q24 = uniforms.q[5].w;
    q25 = uniforms.q[6].x;
    q26 = uniforms.q[6].y;
    q27 = uniforms.q[6].z;
    q28 = uniforms.q[6].w;
    q29 = uniforms.q[7].x;
    q30 = uniforms.q[7].y;
    q31 = uniforms.q[7].z;
    q32 = uniforms.q[7].w;

    // ---- begin translated user body ----
"#;

Fragment-shader header for user-authored warp shaders. Same shape as USER_COMP_FRAGMENT_PREFIX (seed every MD2 var<private> binding from uniforms and per-invocation inputs), but uv/uv_orig come from the rasterised warp UV — not from a fullscreen-triangle screen UV — and ret is seeded from the previous-frame texture at that warped UV. The renderer binds prev_texture to sampler_main_texture so GetPixel(uv) (defined in the shared helpers) does the right thing.