GPU tessellation: the shader's basis slots follow the CPU evaluator's highest degree

tess.wgsl sized its basis arrays for degree 7 by hand. It now holds 17 slots, degree 16 as the
CPU evaluator, and a test fails when the two drift apart again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/gpu/src/tess.wgsl

@@ -7,8 +7,8 @@
 const PLANE: u32 = 0u;
 const NURBS: u32 = 1u;
 const REVOLVED: u32 = 2u;
-// Basis slots for the highest degree (MAX_SPLINE_DEGREE = 7).
-const W: u32 = 8u;
+// Basis slots for the highest degree (MAX_SPLINE_DEGREE = 16).
+const W: u32 = 17u;
 
 struct Surface {
     kind: u32,

crates/gpu/src/tests.rs

@@ -96,6 +96,14 @@ fn the_hlsl_for_fxc_has_no_arrays_fxc_must_unroll_loops_for() {
     }
 }
 
+/// The shader's basis slots hold the highest degree the CPU evaluator takes, so every surface
+/// the CPU evaluates the GPU evaluates too.
+#[test]
+fn the_shader_takes_every_degree_the_cpu_evaluator_does() {
+    let slots = format!("const W: u32 = {}u;", solidcraft_geom::bspline::MAX_SPLINE_DEGREE + 1);
+    assert!(SHADER.contains(&slots), "tess.wgsl must declare `{slots}`");
+}
+
 fn ok<T, E: std::fmt::Debug>(r: std::result::Result<T, E>) -> T {
     match r {
         Ok(v) => v,