fix(desktop): dedupe duplicate snapshots + silence chromium debug noise

- store.ts: per-designId serialized queue in persistArtifactSnapshot
  collapses the race between applyGenerateResult and agent_end paths;
  content-based dedupe drops the second write when parent html matches.
- scripts/dev.cjs: line-filter child stderr to drop Chromium
  'Hit debug scenario: 4' noise from iframe srcdoc navigations
  (electron/electron#44368, upstream not-planned).

Author: hqhq1025

apps/desktop/scripts/dev.cjs

@@ -12,10 +12,38 @@ env.ELECTRON_RUN_AS_NODE = undefined;
 
 const child = spawn(process.execPath, [electronViteBin, 'dev', ...process.argv.slice(2)], {
   env,
-  stdio: 'inherit',
+  stdio: ['inherit', 'inherit', 'pipe'],
   windowsHide: false,
 });
 
+// Chromium emits `ERROR:debug_utils.cc(14)] Hit debug scenario: 4` from iframe
+// srcdoc navigations — a known regression (electron/electron#44368) closed by
+// upstream as "not planned". Harmless but floods the terminal on every preview
+// render, so drop just that line. Everything else passes through untouched.
+const NOISY_STDERR = /Hit debug scenario:\s*4/;
+let stderrTail = '';
+child.stderr.on('data', (chunk) => {
+  const combined = stderrTail + chunk.toString('utf8');
+  const lastNewline = combined.lastIndexOf('\n');
+  const complete = lastNewline >= 0 ? combined.slice(0, lastNewline + 1) : '';
+  stderrTail = lastNewline >= 0 ? combined.slice(lastNewline + 1) : combined;
+  if (complete.length === 0) return;
+  const filtered = complete
+    .split('\n')
+    .filter((line, index, arr) => {
+      if (index === arr.length - 1 && line === '') return true;
+      return !NOISY_STDERR.test(line);
+    })
+    .join('\n');
+  if (filtered.length > 0) process.stderr.write(filtered);
+});
+child.stderr.on('end', () => {
+  if (stderrTail.length > 0 && !NOISY_STDERR.test(stderrTail)) {
+    process.stderr.write(stderrTail);
+  }
+  stderrTail = '';
+});
+
 child.on('exit', (code, signal) => {
   if (signal) {
     process.kill(process.pid, signal);

apps/desktop/src/renderer/src/store.ts

@@ -528,25 +528,47 @@ function artifactFromResult(
   return { type: source.type, content: source.content, prompt, message };
 }
 
+// Per-designId serialization queue. A single generate run reaches this
+// function twice — once from applyGenerateResult → persistDesignState and once
+// from the agent_end handler → persistAgentRunSnapshot. Without serialization
+// both callers race on `snapshots.list`, see zero rows, and both write a fresh
+// parent-less 'initial' snapshot. Chaining per design collapses the race and
+// lets the content-based dedupe below drop the second write cleanly.
+const snapshotPersistLocks = new Map<string, Promise<unknown>>();
+
 async function persistArtifactSnapshot(
   designId: string,
   artifact: PersistArtifact,
 ): Promise<string | null> {
   if (!window.codesign) return null;
-  // Look up the latest snapshot to chain parentId; the first generation in a
-  // design has no parent and uses type='initial', subsequent ones use 'edit'.
-  const existing = await window.codesign.snapshots.list(designId);
-  const parent = existing[0] ?? null;
-  const created = await window.codesign.snapshots.create({
-    designId,
-    parentId: parent?.id ?? null,
-    type: parent ? 'edit' : 'initial',
-    prompt: artifact.prompt,
-    artifactType: toSnapshotArtifactType(artifact.type),
-    artifactSource: artifact.content,
-    ...(artifact.message ? { message: artifact.message } : {}),
+  const prior = snapshotPersistLocks.get(designId) ?? Promise.resolve();
+  const run = prior.then(async () => {
+    if (!window.codesign) return null;
+    const existing = await window.codesign.snapshots.list(designId);
+    const parent = existing[0] ?? null;
+    // Dedupe by content: the agent_end path and the generate-result path both
+    // fire at the tail of a run and often hold identical html. Returning the
+    // existing id avoids duplicate rows without making either caller aware of
+    // the other.
+    if (parent !== null && parent.artifactSource === artifact.content) {
+      return parent.id;
+    }
+    const created = await window.codesign.snapshots.create({
+      designId,
+      parentId: parent?.id ?? null,
+      type: parent ? 'edit' : 'initial',
+      prompt: artifact.prompt,
+      artifactType: toSnapshotArtifactType(artifact.type),
+      artifactSource: artifact.content,
+      ...(artifact.message ? { message: artifact.message } : {}),
+    });
+    return created?.id ?? null;
   });
-  return created?.id ?? null;
+  snapshotPersistLocks.set(
+    designId,
+    run.catch(() => {}),
+  );
+  return run;
 }
 
 /**