fix: print pack and lock errors on stderr

The lock refusal, "no pack for", "no manifest for" and "no pack to
verify" went to stdout, so a run redirected to a log exited 1 with
nothing on the terminal. A test reads both scripts and refuses an ERROR
line printed to stdout.

scripts/generate_pack.py

@@ -2547,13 +2547,13 @@ def _run_manifest_mode(
                         _write_manifest_if_changed(alias_path, alias_manifest)
                         print(f"  {alias_path}: alias of {representative}")
             except (FileNotFoundError, OSError, yaml.YAMLError) as e:
-                print(f"  ERROR: {e}")
+                print(f"  ERROR: {e}", file=sys.stderr)
                 failed.append(representative)
 
     # A platform whose manifest could not be built has none, and install.py
     # serves whatever an older run left there: that is a failed run.
     if failed:
-        print(f"ERROR: no manifest for {', '.join(sorted(set(failed)))}")
+        print(f"ERROR: no manifest for {', '.join(sorted(set(failed)))}", file=sys.stderr)
         sys.exit(1)
 
 
@@ -2564,7 +2564,7 @@ def _pack_output_lock(output_dir: str, exclusive: bool = True) -> Iterator[None]
         with artifact_lock(output_dir, exclusive=exclusive):
             yield
     except ArtifactLockBusy as exc:
-        print(f"ERROR: {exc}")
+        print(f"ERROR: {exc}", file=sys.stderr)
         sys.exit(1)
 
 
@@ -2667,7 +2667,7 @@ def _run_verify_packs(args: argparse.Namespace) -> None:
     if not verified:
         # Every platform skipped is no check at all: a mistyped or purged
         # output directory read as a passed integrity check.
-        print(f"ERROR: no pack to verify in {args.output_dir}")
+        print(f"ERROR: no pack to verify in {args.output_dir}", file=sys.stderr)
         all_ok = False
     if not all_ok:
         sys.exit(1)
@@ -2829,7 +2829,7 @@ def _run_platform_packs(
                         pack_name=group_name,
                     )
             except (FileNotFoundError, OSError, yaml.YAMLError) as e:
-                print(f"  ERROR: {e}")
+                print(f"  ERROR: {e}", file=sys.stderr)
                 failed.append(representative)
             else:
                 # A builder that declined (no system left under the target)
@@ -2867,7 +2867,7 @@ def _run_platform_packs(
     if failed:
         # Verification only sees the ZIPs that exist, so a pack that was never
         # written does not fail it.
-        print(f"ERROR: no pack for {', '.join(sorted(set(failed)))}")
+        print(f"ERROR: no pack for {', '.join(sorted(set(failed)))}", file=sys.stderr)
         sys.exit(1)
     if not all_ok:
         sys.exit(1)

scripts/pipeline.py

@@ -467,7 +467,7 @@ def main() -> None:
         try:
             held.enter_context(hold_artifact_lock(args.output_dir))
         except ArtifactLockBusy as exc:
-            print(f"ERROR: {exc}")
+            print(f"ERROR: {exc}", file=sys.stderr)
             sys.exit(1)
 
     all_ok = True
@@ -655,7 +655,7 @@ def main() -> None:
                     if stale:
                         print(f"Purged {len(stale)} stale pack file(s) from {out_dir}/")
             except ArtifactLockBusy as exc:
-                print(f"ERROR: {exc}")
+                print(f"ERROR: {exc}", file=sys.stderr)
                 sys.exit(1)
 
         pack_cmd = [

tests/test_artifact_lock.py

@@ -108,7 +108,7 @@ class PackLockCliTest(unittest.TestCase):
                 ]
             )
         self.assertEqual(proc.returncode, 1, proc.stdout + proc.stderr)
-        self.assertIn("is in use by another run", proc.stdout)
+        self.assertIn("is in use by another run", proc.stderr)
 
     def test_every_writing_mode_refuses_a_locked_output(self):
         """Two of seven modes took the lock, and five wrote straight into it.
@@ -153,7 +153,7 @@ class PackLockCliTest(unittest.TestCase):
                 ]
             )
         self.assertEqual(proc.returncode, 1, proc.stdout + proc.stderr)
-        self.assertIn("is in use by another run", proc.stdout)
+        self.assertIn("is in use by another run", proc.stderr)
 
     def test_pipeline_refuses_locked_output(self):
         with artifact_lock(self.dir):
@@ -167,7 +167,7 @@ class PackLockCliTest(unittest.TestCase):
                 ]
             )
         self.assertEqual(proc.returncode, 1, proc.stdout + proc.stderr)
-        self.assertIn("is in use by another run", proc.stdout)
+        self.assertIn("is in use by another run", proc.stderr)
 
 
 class HeldLockTest(unittest.TestCase):

tests/test_audit_regressions.py

@@ -5192,7 +5192,7 @@ class ANamedPackThatIsAbsentFails(unittest.TestCase):
                 cwd=ROOT, capture_output=True, text=True, timeout=600,
             )
         self.assertEqual(result.returncode, 1)
-        self.assertIn("no pack to verify", result.stdout)
+        self.assertIn("no pack to verify", result.stderr)
 
 
 
@@ -5945,5 +5945,35 @@ class HashInputNamesWhatItAccepts(unittest.TestCase):
         self.assertEqual(sha1_for_hash("md5", entry["md5"][:29], db), sha1)
 
 
+class ErrorsGoToStderr(unittest.TestCase):
+    """The lock refusal and "no pack for" went to stdout: a run redirected to
+    a log showed nothing on the terminal and exited 1."""
+
+    @staticmethod
+    def _leading_text(node: ast.expr) -> str:
+        if isinstance(node, ast.Constant) and isinstance(node.value, str):
+            return node.value
+        if isinstance(node, ast.JoinedStr) and node.values:
+            return ErrorsGoToStderr._leading_text(node.values[0])
+        return ""
+
+    def test_no_error_line_is_printed_to_stdout(self):
+        for script in ("generate_pack.py", "pipeline.py"):
+            tree = ast.parse((ROOT / "scripts" / script).read_text(encoding="utf-8"))
+            seen = 0
+            for node in ast.walk(tree):
+                if not (isinstance(node, ast.Call) and getattr(node.func, "id", "") == "print"
+                        and node.args):
+                    continue
+                text = self._leading_text(node.args[0])
+                # Detail lines under a pack's own verdict stay with it.
+                if not text.lstrip().startswith("ERROR") or text.startswith("    "):
+                    continue
+                seen += 1
+                with self.subTest(script=script, line=node.lineno):
+                    self.assertIn("file", [k.arg for k in node.keywords])
+            self.assertGreater(seen, 0, script)
+
+
 if __name__ == "__main__":
     unittest.main()