feat(ui): JSON에 nextSep/childSep/nameNewline 포함 및 복원

toJSON() serNode에 nextSep, childSep, nameNewline 필드 추가.
parseJsonSection에서 해당 필드 읽어 노드에 복원.
JSON slim/full 양쪽 모두 포함됨 (값이 null이 아니므로 stripNulls 통과).

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

ui/builder.html

@@ -852,7 +852,10 @@ function toJSON(slim=false){
       footer:node.settings.footer??null,
       prefix:node.settings.prefix??null,
       suffix:node.settings.suffix??null,
-      attributes:attrs,children
+      attributes:attrs,children,
+      nextSep:node.nextSep??'blank',
+      childSep:node.childSep??'blank',
+      nameNewline:node.nameNewline??true,
     });
     depth--;
     return r;
@@ -957,6 +960,10 @@ function parseJsonSection(sec){
   const mark=sec.attributes?.mark??'';
   const n=mkNode(mark, sec.content??'');
 
+  if(sec.nextSep!==undefined)    n.nextSep=sec.nextSep;
+  if(sec.childSep!==undefined)   n.childSep=sec.childSep;
+  if(sec.nameNewline!==undefined) n.nameNewline=sec.nameNewline;
+
   if(sec.header) n.settings.header=sec.header;
   if(sec.footer) n.settings.footer=sec.footer;
   if(sec.prefix) n.settings.prefix=sec.prefix;