Loose grains and solid ground look different: a seam on the rock, pebbly lighter grains (t33)

- Smooth look: natural ground gets a soft seam where loose grains meet it; packed grains keep a
  pebble texture and are slightly lighter. Pixel look and thermal lens: the same idea per pixel.
- play.py: the F-on-a-mold toast check records toasts the key press makes instead of counting
  the ones on screen, which failed when an older toast expired at the same moment.

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

src/render/smooth-shaders.js

@@ -176,9 +176,11 @@ void main() {
     float band = 1.0 - smoothstep(0.5, 0.9, M);
     float lit = dot(nrm, normalize(vec2(-0.5, -1.0)));
     tc *= 1.0 + 0.42 * lit * band;
-    // the outline follows where ground and grains meet open space, never where rock meets gravel
+    // Keep the air outline; a softer, screen-width seam also separates rock from loose grains.
     float dm = (M - 0.5) / fwM;
     tc *= mix(0.4, 1.0, smoothstep(0.5, 1.4 + zp * 0.08, dm));
+    float dt = (F.r - 0.5) / fwT;
+    if (nc == 1 && pw > 0.0) tc *= mix(0.65, 1.0, smoothstep(0.0, 1.5, dt));
     C = mix(C, tc, smoothstep(-0.6, 0.6, (F.r - 0.5) / fwT));
   }
 
@@ -200,10 +202,13 @@ void main() {
     }
     float cover = np == 4 ? 1.0 : 1.0 - smoothstep(1.0 - px / gr, 1.0 + px / gr, best);
     if (closed && F.r < 0.5) cover = 1.0;
+    // Pebbles stop at the rock face so they cannot paint over its thin seam.
+    cover *= 1.0 - smoothstep(-0.6, 0.6, (F.r - 0.5) / fwT);
     if (cover > 0.0) {
       float r2 = min(1.0, dot(gd, gd));
       vec3 n3 = normalize(vec3(gd * 0.85, sqrt(1.0 - r2) + 0.25));
-      float sh = np == 4 ? 0.9 + 0.08 * vn(w * 0.9) : 0.8 + 0.3 * max(0.0, dot(n3, normalize(vec3(-0.45, -0.8, 0.6))));
+      float sh = 0.98 + 0.18 * max(0.0, dot(n3, normalize(vec3(-0.45, -0.8, 0.6))));
+      sh *= mix(1.0, 0.94, smoothstep(0.7 - px * 0.2, 1.15 + px * 0.2, best));
       vec3 pc = (np == 4 ? (k0.rgb * wq.x + k1.rgb * wq.y + k2.rgb * wq.z + k3.rgb * wq.w) : gc) * sh;
       if (!closed) pc *= mix(1.0, 0.72, smoothstep(0.7, 1.0, best));
       C = mix(C, pc, cover);