fix some cython code

.cursor/rules/python.mdc

@@ -1,6 +1,6 @@
 ---
 description: "Python coding conventions: PEP 8, type hints, pydantic, pytest, async patterns, project structure"
-globs: ["**/*.py", "**/pyproject.toml", "**/requirements*.txt"]
+globs: ["**/*.py", "**/*.pyx", "**/*.pxd", "**/pyproject.toml", "**/requirements*.txt"]
 ---
 # Python
 
@@ -12,5 +12,8 @@ globs: ["**/*.py", "**/pyproject.toml", "**/requirements*.txt"]
 - Catch specific exceptions, never bare `except:`; use custom exception classes
 - Use `async`/`await` with `asyncio` for I/O-bound concurrency
 - Use `pytest` for testing (not `unittest`); fixtures for setup/teardown
-- Use virtual environments (`venv` or `poetry`); pin dependencies
+- **NEVER install packages globally** (`pip install` / `pip3 install` without a venv). ALWAYS use a virtual environment (`venv`, `poetry`, or `conda env`). If no venv exists for the project, create one first (`python3 -m venv .venv && source .venv/bin/activate`) before installing anything. Pin dependencies.
 - Format with `black`; lint with `ruff` or `flake8`
+
+## Cython
+- In `cdef class` methods, prefer `cdef` over `cpdef` unless the method must be callable from Python. `cdef` = C-only (fastest), `cpdef` = C + Python, `def` = Python-only. Check all call sites before choosing.