Live: stop the preflight writing source, cache the resolution

The polling-rework preflight wrote the variant scaffold into source during the
poll lease, before the agent acted. On source-preview targets (React/Vue/Vite,
everything but the svelte-component path) that write full-reloaded the
framework; a browser caught mid-reload missed the agent's variant write and the
SSE done, and sat stranded at 0/N.

Restore the 3.5 single-atomic-edit semantics: the preflight still resolves the
element location and computes the scaffold, but --defer-source-write leaves
source untouched and hands the agent the wrapper text plus the picked source
range. The agent splices variants into the wrapper and replaces the range in
one write, so the framework reloads exactly once. The svelte-component path is
untouched (it never writes route source). The missed-completion recovery stays
as defense in depth.

Also cache the resolved source file per target signature (locator + route):
the ~7.6s tree search re-ran on every generate for the same element; a hit now
points the helper straight at the file via --file, invalidated when the target
changes or a resolution fails.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/live-e2e/agent.mjs

@@ -1292,14 +1292,12 @@ function renderVariantsBlock({ sessionId, indent, output, commentSyntax, file, s
 }
 
 /**
- * Read the wrapped file, find the "insert below this line" marker, splice in
- * the rendered variants block, write back.
+ * Splice the rendered variants block into an array of wrapper lines at the
+ * "insert below this line" marker. Pure: returns the new lines array. Used
+ * both against a whole file (wrapper already in source) and against a
+ * standalone wrapper block (deferred source write, agent writes it now).
  */
-async function spliceVariantsIntoWrapper({ tmp, wrapInfo, sessionId, output }) {
-  const filePath = path.join(tmp, wrapInfo.file);
-  const src = await fs.readFile(filePath, 'utf-8');
-  const lines = src.split('\n');
-
+function spliceVariantsIntoLines(lines, { sessionId, output, commentSyntax, file, styleMode }) {
   // Find the "Variants: insert below this line" comment line — definitive
   // marker, robust to any indentation off-by-one. Matches in any comment
   // style (HTML / JSX / Astro).
@@ -1307,7 +1305,7 @@ async function spliceVariantsIntoWrapper({ tmp, wrapInfo, sessionId, output }) {
     l.includes('Variants: insert below this line'),
   );
   if (markerIdx === -1) {
-    throw new Error('insert marker not found in ' + wrapInfo.file);
+    throw new Error('insert marker not found in ' + file);
   }
 
   const indent = (lines[markerIdx].match(/^\s*/) || [''])[0];
@@ -1320,26 +1318,73 @@ async function spliceVariantsIntoWrapper({ tmp, wrapInfo, sessionId, output }) {
     sessionId,
     indent: wrapperIndent,
     output,
-    commentSyntax: wrapInfo.commentSyntax,
-    file: wrapInfo.file,
-    styleMode: wrapInfo.styleMode,
+    commentSyntax,
+    file,
+    styleMode,
   });
 
   const endMarkerIdx = lines.findIndex((line, index) =>
     index > markerIdx && line.includes('impeccable-variants-end ' + sessionId),
   );
   if (endMarkerIdx === -1) {
-    throw new Error('end marker not found in ' + wrapInfo.file);
+    throw new Error('end marker not found in ' + file);
   }
-  const tailIdx = wrapInfo.commentSyntax.open === '{/*'
+  const tailIdx = commentSyntax.open === '{/*'
     ? endMarkerIdx
     : endMarkerIdx - 1;
 
-  const next = [
+  return [
     ...lines.slice(0, markerIdx + 1),
     block,
     ...lines.slice(tailIdx),
   ];
+}
+
+/**
+ * Read the wrapped file, find the "insert below this line" marker, splice in
+ * the rendered variants block, write back. Used when the wrapper is already
+ * present in source (agent's own wrap fallback, no preflight).
+ */
+async function spliceVariantsIntoWrapper({ tmp, wrapInfo, sessionId, output }) {
+  const filePath = path.join(tmp, wrapInfo.file);
+  const src = await fs.readFile(filePath, 'utf-8');
+  const lines = src.split('\n');
+  const next = spliceVariantsIntoLines(lines, {
+    sessionId,
+    output,
+    commentSyntax: wrapInfo.commentSyntax,
+    file: wrapInfo.file,
+    styleMode: wrapInfo.styleMode,
+  });
+  await fs.writeFile(filePath, next.join('\n'), 'utf-8');
+}
+
+/**
+ * Deferred source write (preflight computed the scaffold but left source
+ * untouched). Splice the variants into the scaffold's `wrapperBlock`, then
+ * replace the picked element's source range with the result in ONE write —
+ * the 3.5 atomic single-edit semantics. `replaceEndLine < replaceStartLine`
+ * expresses a pure insertion (insert mode).
+ */
+async function writeDeferredWrapperWithVariants({ tmp, wrapInfo, sessionId, output }) {
+  const filePath = path.join(tmp, wrapInfo.file);
+  const src = await fs.readFile(filePath, 'utf-8');
+  const lines = src.split('\n');
+  const wrapperLines = String(wrapInfo.wrapperBlock).split('\n');
+  const splicedWrapper = spliceVariantsIntoLines(wrapperLines, {
+    sessionId,
+    output,
+    commentSyntax: wrapInfo.commentSyntax,
+    file: wrapInfo.file,
+    styleMode: wrapInfo.styleMode,
+  });
+  const startIdx = wrapInfo.replaceStartLine - 1;
+  const endIdx = wrapInfo.replaceEndLine - 1; // may be startIdx-1 for insertion
+  const next = [
+    ...lines.slice(0, startIdx),
+    ...splicedWrapper,
+    ...lines.slice(endIdx + 1),
+  ];
   await fs.writeFile(filePath, next.join('\n'), 'utf-8');
 }
 
@@ -1661,6 +1706,8 @@ export async function runAgentLoop({
         trace('agent.write.start', { id: event.id, file: wrapInfo.file });
         if (wrapInfo.previewMode === 'svelte-component') {
           await writeSvelteComponentVariants({ tmp, wrapInfo, event, output, writeParams: true });
+        } else if (wrapInfo.sourceWritten === false) {
+          await writeDeferredWrapperWithVariants({ tmp, wrapInfo, sessionId: event.id, output });
         } else {
           await spliceVariantsIntoWrapper({ tmp, wrapInfo, sessionId: event.id, output });
         }