fix: colab bench manifest — entities must be dicts with char spans

Bench clip failed in Colab because entity_retention calls ent.get() but
the manifest used plain strings -> AttributeError -> clip counted as
failure (failure_rate 1.0). Notebook now builds entities as
{canonical, surface, start_char, end_char} from the reference text and
writes a reference file for meaningful metrics.

scripts/build_colab_notebook.py

@@ -332,16 +332,31 @@ def cells() -> list[dict]:
         ),
         code(
             "# 13) 벤치마크 — 샘플 manifest 사용 (선택, 시간 소요)\n"
-            "# 간단 manifest 생성\n"
+            "# entities는 {canonical, surface, start_char, end_char} dict여야 한다\n"
+            "# (문자열이면 entity_retention이 .get() 호출에 실패해 clip이 실패 처리됨)\n"
             "import yaml\n"
+            "\n"
+            "REF_TEXT = '오늘은 vLLM 서버를 Kubernetes 클러스터에 배포하는 방법을 설명합니다. GPU 가속 추론으로 대기 시간을 줄일 수 있습니다.'\n"
+            "with open('/content/reference.txt', 'w', encoding='utf-8') as f:\n"
+            "    f.write(REF_TEXT)\n"
+            "\n"
+            "entities = []\n"
+            "for name in ('vLLM', 'Kubernetes', 'GPU'):\n"
+            "    idx = REF_TEXT.index(name)\n"
+            "    entities.append({\n"
+            "        'canonical': name, 'surface': name,\n"
+            "        'start_char': idx, 'end_char': idx + len(name),\n"
+            "    })\n"
+            "\n"
             "manifest = {\n"
             "    'name': 'colab-quick',\n"
             "    'dataset_version': '1.0',\n"
             "    'language': 'ko',\n"
             "    'targets': {'entity_preservation': 0.95, 'cer': 0.15},\n"
             "    'clips': [\n"
-            "        {'name': 'ko-en-tech', 'audio_path': 'samples/colab-ko-en.mp3',\n"
-            "         'duration_sec': 10.5, 'entities': ['vLLM', 'Kubernetes', 'GPU']},\n"
+            "        {'id': 'ko-en-tech', 'audio_path': 'samples/colab-ko-en.mp3',\n"
+            "         'reference_path': '/content/reference.txt',\n"
+            "         'duration_sec': 10.5, 'entities': entities},\n"
             "    ],\n"
             "}\n"
             "yaml.safe_dump(manifest, open('/content/manifest.yaml', 'w'))\n"