Fewer, smaller messages: mold percentages, markers for machine heat, short status lines

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

src/sim/blocks.js

@@ -8,7 +8,7 @@ function canPlaceBlock(bx, by) {
   if (blockP[by * BW + bx]) return 'Something is already built there';
   for (const i of blockCellList(bx, by)) {
     if (mach[i] || part[i]) return 'Something is already built there';
-    if (PH[mat[i]] === PH_SOLID) return 'Needs open space. Clear the rock first.';
+    if (PH[mat[i]] === PH_SOLID) return 'Needs open space; clear the rock first.';
     if (occ[i]) return 'A bar is in the way';
   }
   return '';