fix: merge corrected TF files instead of replacing all

- fmTf now merges new code blocks with existing S.tfFiles: updated files
  are replaced by name, untouched files are preserved, new files are added
- System prompt changed: model generates ONLY the files that need fixing
  instead of all 25 files, drastically reducing output size
- Prevents model from discarding existing work when correcting errors

frontend/index.html

@@ -736,15 +736,31 @@ function fmTf(t){
     return '<div class="tf-plan-block"><div class="tf-plan-header">'+TF_ICON+' Resource Plan — '+S.tfPlan.length+' resource(s)</div><div class="tf-plan-list">'+
       S.tfPlan.map(x=>'<div class="tf-plan-item"><span class="tf-plan-add">+</span><span class="tf-plan-type">'+escHtml(x.type)+'</span><span class="tf-plan-desc">'+escHtml(x.desc)+'</span></div>').join('')+'</div></div>';
   });
-  // Build files list — merge blocks with same filename, keep separate files
+  // Build files list — merge new blocks with existing files (corrections preserve untouched files)
   if(codeBlocks.length){
-    const fileMap={};
+    const newMap={};
     codeBlocks.forEach(b=>{
-      if(fileMap[b.name])fileMap[b.name]+='\n\n'+b.content;
-      else fileMap[b.name]=b.content;
+      if(newMap[b.name])newMap[b.name]+='\n\n'+b.content;
+      else newMap[b.name]=b.content;
     });
-    S.tfFiles=Object.entries(fileMap).map(([name,content])=>({name,content,size:content.length,type:'hcl'}));
-    // Set tfCode to combined content for backward compat
+    // If existing files: merge (update matched, keep unmatched)
+    if(S.tfFiles.length){
+      const merged=[];
+      const usedNew=new Set();
+      // Update existing files with new content if name matches
+      for(const f of S.tfFiles){
+        if(newMap[f.name]!=null){merged.push({name:f.name,content:newMap[f.name],size:newMap[f.name].length,type:'hcl'});usedNew.add(f.name)}
+        else{merged.push(f)}
+      }
+      // Add any truly new files
+      for(const[name,content] of Object.entries(newMap)){
+        if(!usedNew.has(name))merged.push({name,content,size:content.length,type:'hcl'})
+      }
+      S.tfFiles=merged;
+    }else{
+      // First generation: set all
+      S.tfFiles=Object.entries(newMap).map(([name,content])=>({name,content,size:content.length,type:'hcl'}));
+    }
     S.tfCode=S.tfFiles.map(f=>'// filename: '+f.name+'\n'+f.content).join('\n\n');
   }
   r=r.replace(/\*\*(.*?)\*\*/g,'<strong>$1</strong>').replace(/`(.*?)`/g,'<code>$1</code>').replace(/\n/g,'<br>');