fix: add debug for npm exec in _pick_yarn_cmd

npm_install.py

@@ -2006,13 +2006,22 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
         # Use npm exec directly (bypasses broken corepack proxy)
         if shutil.which("npm"):
             try:
+                result = subprocess.run(
+                    ["npm", "exec", "--yes", "yarn@stable", "--", "--version"],
+                    capture_output=True, text=True, timeout=30,
+                )
+                if DEBUG:
+                    print(f"    debug pick: npm exec version={result.stdout.strip()} rc={result.returncode}")
                 major = _get_yarn_major_version(["npm", "exec", "--yes", "yarn@stable", "--"])
                 if major and major >= 4:
                     if DEBUG:
                         print(f"    debug pick: using npm exec for yarn")
                     return ["npm", "exec", "--yes", "yarn@stable", "--"]
-            except Exception:
+            except Exception as e:
+                if DEBUG:
+                    print(f"    debug pick: npm exec failed: {e}")
                 pass
+        return None
 
     def _cleanup_yarn_artifacts():
         """Remove corrupted yarn artifacts."""