Sync .cursor from detections

.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 +12,10 @@ 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.
+- **Python cannot call `cdef` methods.** If a `.py` file needs to call a `cdef` method on a Cython object, there are exactly two options: (a) convert the calling file to `.pyx`, `cimport` the class, and use a typed parameter so Cython dispatches the call at the C level; or (b) change the method to `cpdef` if it genuinely needs to be callable from both Python and Cython. Never leave a bare `except Exception: pass` around such a call — it will silently swallow the `AttributeError` and make the failure invisible for a very long time.
+- When converting a `.py` file to `.pyx` to gain access to `cdef` methods: add the new extension to `setup.py`, add a `cimport` of the relevant `.pxd`, type the parameter(s) that carry the Cython object, and delete the old `.py` file. This ensures the cross-language call is resolved at compile time, not at runtime.