Diagonal screw tubes, laid by a 45° drag (t35)

- Tubes run in eight directions: a drag at about 45° lays a diagonal run, R cycles all eight.
- Diagonal runs carry grains at the straight rate (120 grains/s), join straight tubes and belts,
  keep grains in at the corners, pass water, and carry bars level.
- New part ids 11-14 (saved ids are never renumbered); tube checks use an IS_TUBE table.
- New test/t_diag_tubes.js (85 checks).

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

src/render/smooth-shaders.js

@@ -245,11 +245,16 @@ void main() {
       C = mix(C, bc, inside);
     }
   }
-  else if (nc >= 9 && nc <= 12) {
+  else if ((nc >= 9 && nc <= 12) || (nc >= 19 && nc <= 22)) {
     // screw tube: a copper pipe; what is inside shows through, and the screw's flights turn
     bool vert = nc == 9 || nc == 11;
     float sp2 = nc == 9 ? -1.0 : nc == 10 ? 1.0 : nc == 11 ? 1.0 : -1.0;
     float a = (vert ? fract(w.x * 0.25) : fract(w.y * 0.25)) * 4.0, along = vert ? w.y : w.x;
+    if (nc >= 19) {
+      vec2 d = nc == 19 ? vec2(1.0, -1.0) : nc == 20 ? vec2(1.0, 1.0) : nc == 21 ? vec2(-1.0, 1.0) : vec2(-1.0, -1.0);
+      a = fract(dot(w, vec2(-d.y, d.x)) * 0.25) * 4.0;
+      along = dot(w, d); sp2 = 1.0;
+    }
     float wall = 0.62;
     vec3 tc;
     if (a < wall || a > 4.0 - wall) {