fix: stabilize S-52 sprite registration

web/kyushu_compare.html

@@ -667,7 +667,13 @@ async function loadS52SpriteAtlas(){
     });
     return {metadata:metadata,image:image};
   })();
-  return s52SpriteAtlasPromise;
+  try{
+    return await s52SpriteAtlasPromise;
+  }catch(err){
+    // Do not permanently cache a transient fetch/image failure.
+    s52SpriteAtlasPromise=null;
+    throw err;
+  }
 }
 
 async function ensureS52SpriteImages(map){
@@ -726,7 +732,9 @@ function prepareS52Style(raw){
   source.maxzoom=12;
   source.bounds=[128.0,29.5,132.5,34.8];
   source.attribution="CM95 九州当前 S-52 Production";
-  style.sprite=location.origin + "/s52/sprite";
+  // Dynamic setStyle() and MapLibre's automatic sprite loader are unreliable
+  // in this comparison page.  Register S-52 images explicitly after style.load.
+  delete style.sprite;
   if(referenceStyleTemplate && referenceStyleTemplate.glyphs){
     style.glyphs=referenceStyleTemplate.glyphs;
   }
@@ -745,6 +753,9 @@ async function ensureS52StyleTemplate(){
 async function applyLeftStyleMode(){
   if(debugLeftS52.checked){
     setState(leftState,"加载完整 S-52 style…","");
+    // Prefetch the atlas before setStyle so style.load only has to register
+    // already-local pixels; this avoids transient fetch failures mid-style swap.
+    await loadS52SpriteAtlas();
     const raw=await ensureS52StyleTemplate();
     const style=prepareS52Style(raw);
     activeLeftStyleTemplate=style;