Sloped walls with a flat 45° face, and diagonal wall lines (t36)

- New Sloped wall piece: a half-block triangle in any wall material, at half a wall's cost.
  Dragging walls or slopes at about 45° lays a diagonal line; the drag picks the slope's
  orientation and T flips or turns it.
- Each slope fills one cell past its corner, so a diagonal run is sealed against grains and water.
- New test/t_slopes.js: a slope funnel passes 400/400 sand (square steps 379/400).

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

src/render/smooth-shaders.js

@@ -271,7 +271,7 @@ void main() {
     }
     C = tc;
   }
-  else if (nc == 6) {
+  else if (nc == 6 || (nc >= 15 && nc <= 18)) {
     vec2 lb = fract(w * 0.25) * 4.0, rb = 4.0 - lb;
     float e = min(min(lb.x, lb.y), min(rb.x, rb.y));
     vec3 tc = own.rgb * (0.92 + 0.14 * fbm(w * 0.55));
@@ -279,7 +279,14 @@ void main() {
     float shd = max(1.0 - smoothstep(0.0, 0.8, rb.x), 1.0 - smoothstep(0.0, 0.8, rb.y));
     tc *= 1.0 + 0.2 * hl - 0.26 * shd;
     tc *= mix(0.5, 1.0, smoothstep(px * 0.5, px * 0.5 + max(px * 1.2, 0.14), e));
-    C = tc;
+    if (nc >= 15) {
+      vec2 sl = lb;
+      if (nc == 16 || nc == 18) sl.x = 4.0 - sl.x;
+      if (nc >= 17) sl.y = 4.0 - sl.y;
+      float d = (sl.y - sl.x) * 0.70710678;
+      tc *= 1.0 + 0.2 * (1.0 - smoothstep(0.0, 0.6, d));
+      C = mix(C, tc, smoothstep(-px * 0.5, px * 0.5, d));
+    } else C = tc;
   }
 
   else if (nc == 13 || nc == 14) {