fix: threading.Lock on LandmarkExtractor + composer prompt format match

Two deep-check IMPORTANTs:
- B.F2 / A.F4: Gradio multi-threaded server can race the lazy
initialisation of _extractor_singleton; concurrent first-clicks
could build two MediaPipe instances. threading.Lock with
double-checked-locking pattern fixes it.
- C.F6: composer user prompt previously sent ', '.join(signs) while
the system prompt teaches the model with Python list syntax. Switch
user prompt to repr(list(signs)) so format matches examples — should
improve the 'My name is Lucas' hero demo.

signbridge/composer/sentence.py

@@ -110,7 +110,9 @@ def compose_sentence(signs: Sequence[str]) -> str:
         return ""
 
     client, model = _resolve_client()
-    user_prompt = "ASL signs: " + ", ".join(signs)
+    # System prompt examples use Python-list syntax (e.g. ["L","U","C","A","S"]);
+    # match that format here so the LLM sees inputs and examples consistently.
+    user_prompt = f"ASL signs: {list(signs)!r}"
 
     if client is None:
         return _naive_join(signs)

signbridge/space.py

@@ -17,6 +17,7 @@ from __future__ import annotations
 
 import logging
 import os
+import threading
 from dataclasses import dataclass, field
 
 import gradio as gr
@@ -108,13 +109,22 @@ def _recognize(frame: np.ndarray) -> tuple[str, float]:
 
 
 _extractor_singleton: LandmarkExtractor | None = None
+_extractor_lock = threading.Lock()
 
 
 def _shared_extractor() -> LandmarkExtractor:
+    """Return the lazy-loaded MediaPipe Holistic extractor.
+
+    Double-checked locking so concurrent first-call requests under
+    Gradio's worker threads don't race and build two extractors.
+    """
     global _extractor_singleton
-    if _extractor_singleton is None:
-        _extractor_singleton = LandmarkExtractor()
-    return _extractor_singleton
+    if _extractor_singleton is not None:
+        return _extractor_singleton
+    with _extractor_lock:
+        if _extractor_singleton is None:
+            _extractor_singleton = LandmarkExtractor()
+        return _extractor_singleton
 
 
 def _capture_sign(