Removing slopes no longer leaves unowned seal cells; loading clears them

A slope's seal can sit in a cell its neighbour leaves open (a slope's
open corner, or under a grate, screw or screen), but removal only
cleared it when the neighbour was a sloped wall, so cutting a sloped
belt next to a sloped filter or grate could leave a one-cell belt that
nothing owned and nothing could remove. Removal now asks whether the
neighbour fills that cell itself, and loading a world clears belt and
screen cells that no block or seal owns.

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

src/sim/save.js

@@ -119,6 +119,7 @@ function deserializeWorld(meta, buffers) {
   seen = new Uint8Array(NMAT); meta.seen.forEach((v, k) => { if (k < NMAT) seen[k] = v; });
   ledger = meta.ledger || {};
   Object.assign(stats, meta.stats || {});
+  clearStraySeals();
   powerDirty = true; rebuildPower(); slopeCleatsTick(false); updateRoof();
   if (!weather.base) weather.base = worldWater(); // older saves: take the water it has now as its starting amount
   if (!meta.specks) clearHardSpecks(true); // older saves: tidy away tiny specks of too-hard rock once