Spike raw-syscall native ring lifecycle

[bright2227]

What changed

• add an importable uringloop._uringcore CPython extension
• add a native Ring type that owns io_uring_setup, SQ/CQ mappings, SQE mappings, and deterministic teardown
• expose an ABI version and typing stub
• package the C source in sdists and the extension in wheels
• document this as the raw-syscall route spike and keep the existing CFFI proactor unchanged

Why

Phase 1 of the roadmap requires evaluating raw kernel syscalls against a statically linked vendored liburing backend before choosing the production native core. This is the smallest useful first spike: it establishes and tests the ring lifecycle boundary without mixing in submission, completion, or native request-state work.

Impact

There is no user-facing event-loop behavior change. The current CFFI implementation remains the active backend and behavioral oracle. The new _uringcore extension does not dynamically link to liburing; the static-liburing counterpart and final decision record remain follow-up work.

Validation

• x86_64-linux-gnu-gcc -fsyntax-only -Wall -Wextra -Werror -I/usr/include/python3.12 src/uringcore.c
• .venv/bin/ruff check setup.py tests/unit/test_uringcore.py uringloop/_uringcore.pyi
• .venv/bin/ruff format --check setup.py tests/unit/test_uringcore.py uringloop/_uringcore.pyi
• .venv/bin/pytest tests/ -v — 21 passed
• uv build --wheel --sdist
• ldd uringloop/_uringcore.cpython-312-x86_64-linux-gnu.so — libc only; no liburing dependency