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

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

src/sim/undo.js

@@ -17,8 +17,8 @@ function undoLast() {
   const rec = undoStack.pop();
   if (!rec) return 'Nothing to undo.';
   const prev = undoRec; undoRec = null;
-  let fails = 0, why = '';
-  const fail = (r) => { fails++; if (r && !why) why = r; };
+  let fails = 0;
+  const fail = () => { fails++; };
   for (let k = rec.ops.length - 1; k >= 0; k--) {
     const o = rec.ops[k];
     if (o.t === 'block+') { if (blockP[o.by * BW + o.bx] === o.code) removeBlock(o.bx, o.by); }
@@ -38,7 +38,7 @@ function undoLast() {
   undoRec = prev;
   powerDirty = true;
   const n = rec.ops.length;
-  return fails ? `Undid ${rec.label}, but ${fails === n ? 'none' : fails} of it could not be put back${why ? ': ' + why.replace(/\.$/, '').toLowerCase() : ''}.` : `Undid ${rec.label}.`;
+  return fails ? `Undo incomplete: ${fails} of ${n} changes could not be put back.` : `Undid ${rec.label}.`;
 }
 function clearAllGhosts() { ghostBlocks.clear(); ghostMachines = []; ghostMolds = []; ghostBlocked.clear(); }
 let ghostMissing = 0, ghostIter = null;