chore: cut 1.2.4 and date its changelog entry

README first, since packaging freezes whatever it says at build time:
version pin bumped, and the capability table now mentions that an open
circuit can wait as well as fail fast. Verified the numeric claims by
measurement rather than memory -- record_llm_call still takes 24 fields,
schema.COLUMNS still has 24, meta still caps at 16 keys.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "polygateway"
-version = "1.2.3"
+version = "1.2.4"
 description = "PolyGateway：实验室统一的大语言模型（LLM/VLM/OCR）调度与中转库——多源、限流、重试、熔断、缓存、遥测"
 # registry 包页面的正文只认这一项:缺了页面就是一片空白(1.1.2 的教训,twine 会警告
 # long_description missing 但不阻塞上传)。README 在打包时被固化进产物,发布后再改无效。