Fix demo game fog layer accumulation and web IDBFS mkdir race

- Add stairs position to occupied set to prevent enemy/item overlap
- Remove old fog layer before creating new one on level transitions
- Reset fog_layer reference on new game to avoid stale grid reference
- Wrap FS.mkdir('/save') in try/catch for page reload resilience

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/index.html

@@ -362,7 +362,7 @@
 
     var Module = {
         preRun: [function() {
-            FS.mkdir('/save');
+            try { FS.mkdir('/save'); } catch(e) { /* already exists */ }
             FS.mount(IDBFS, {}, '/save');
             Module.addRunDependency('idbfs-restore');
             FS.syncfs(true, function(err) {