fix(annotate): lock-protect per-line writes for parallel server streams

8 server-streaming threads writing chars unsynchronized cause UTF-8
sequences from different servers to interleave mid-byte, garbling the
terminal output. Switch to line-buffered reads with a single shared
print lock — output stays readable, ready-marker detection still works
on the line containing 'Uvicorn running' / 'Application startup
complete'.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lerobot/annotations/steerable_pipeline/vlm_client.py

@@ -456,6 +456,9 @@ def _spawn_parallel_inference_servers(config: VlmConfig) -> list[str]:
     procs: list[subprocess.Popen] = []
     ready_events: list[threading.Event] = []
     ready_markers = ("Uvicorn running", "Application startup complete")
+    # Single lock for all server-stream threads so multibyte chars from
+    # different servers don't interleave and tear UTF-8 sequences.
+    print_lock = threading.Lock()
 
     base_cmd = config.serve_command or (
         f"vllm serve {shlex.quote(config.model_id)} "
@@ -489,24 +492,17 @@ def _spawn_parallel_inference_servers(config: VlmConfig) -> list[str]:
         ready_events.append(ready)
 
         def _stream(idx: int, p: subprocess.Popen, ev: threading.Event) -> None:
+            # Read whole lines and emit each line atomically under the
+            # shared print_lock so output from N servers stays readable.
             assert p.stdout is not None
-            buf = ""
-            tag_emitted = False
-            while True:
-                ch = p.stdout.read(1)
-                if ch == "":
-                    return
-                if not tag_emitted:
-                    sys.stdout.write(f"[server-{idx}] ")
-                    tag_emitted = True
-                sys.stdout.write(ch)
-                sys.stdout.flush()
-                buf += ch
-                if ch in ("\n", "\r"):
-                    if any(m in buf for m in ready_markers):
-                        ev.set()
-                    buf = ""
-                    tag_emitted = False
+            for line in iter(p.stdout.readline, ""):
+                with print_lock:
+                    sys.stdout.write(f"[server-{idx}] {line}")
+                    if not line.endswith(("\n", "\r")):
+                        sys.stdout.write("\n")
+                    sys.stdout.flush()
+                if any(m in line for m in ready_markers):
+                    ev.set()
 
         threading.Thread(target=_stream, args=(i, proc, ready), daemon=True).start()