build: Python 基线降至 3.12，CI 改 3.12/3.13 双版本矩阵

PEP 695 type 语句、typing.override 等既有写法本就 3.12 可用，代码零
改动；pyproject/ruff/pyright 目标版本同步 py312，README 与 GUIDE 的
版本要求更新，uv.lock 随 requires-python 重新解析。

pyproject.toml

@@ -8,7 +8,7 @@ readme = "README.md"
 authors = [
     { name = "snowy-yang", email = "102860110+snowy-yang@users.noreply.github.com" },
 ]
-requires-python = ">=3.13"
+requires-python = ">=3.12"
 dependencies = [
     "aiohttp>=3.14.3",
     "cryptography>=50.0.1",
@@ -33,7 +33,7 @@ testpaths = ["tests"]
 
 
 [tool.ruff]
-target-version = "py313"
+target-version = "py312"
 line-length = 88
 indent-width = 4
 
@@ -71,7 +71,7 @@ line-ending = "auto"
 
 [tool.pyright]
 typeCheckingMode = "standard"
-pythonVersion = "3.13"
+pythonVersion = "3.12"
 include = ["src"]
 exclude = ["**/__pycache__", "**/.*", "**/node_modules", ".venv"]
 reportMissingImports = true