docs: two stale references in the ground material — the fade's name, and why two materials may share one cache key

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/engine/terrain.ts

@@ -334,9 +334,10 @@ function groundDetailTexture(): THREE.DataTexture {
   texture.magFilter = THREE.LinearFilter;
   // Mipmapped for the reason `swellNormalTexture` states: the mip chain of a
   // normal map converges on flat, so the ground a long way off stops perturbing
-  // its normal without anything having to decide when — and `GROUND_DETAIL_FAR_M`
-  // then finishes the job by taking the fetch's *result* to zero rather than
-  // leaving a residue of the coarsest mip on the horizon.
+  // its normal without anything having to decide when — and the fade at
+  // `GROUND_DETAIL_NOISE_PX` then finishes the job by taking the fetch's
+  // *result* to zero rather than leaving a residue of the coarsest mip on the
+  // horizon.
   texture.minFilter = THREE.LinearMipmapLinearFilter;
   texture.generateMipmaps = true;
   // Eight, as the sea has: from a map pose the ground is seen at a grazing
@@ -537,10 +538,17 @@ mapN.xy *= normalScale * groundFlat * groundSeen;
 normal = normalize( tbn * mapN );`,
         );
     };
-    // Two materials that compile to different programs must not share a cache
-    // key, and `onBeforeCompile` is invisible to three's default key. Set only
-    // on the patched branch: an unpatched ground is an ordinary standard
-    // material and must go on sharing whatever key three gives it.
+    /*
+     * `onBeforeCompile` is invisible to three's default key, so a patched
+     * material that does not declare one gets handed somebody else's program.
+     *
+     * The terrain and the shore plate both answer `"tera:ground"` and that is
+     * correct rather than a collision: `WebGLPrograms.getProgramCacheKey`
+     * appends this string to the *whole* parameter list, and `vertexColors`
+     * is in that list — so the two still hash apart, and two boards of the same
+     * kind still share. Set only on the patched branch: an unpatched ground is
+     * an ordinary standard material and must go on sharing three's own key.
+     */
     material.customProgramCacheKey = () => "tera:ground";
   }
   return material;