vendor catch22 C sources via git submodule instead of copying

[joshuabmoore]

pycatch22 currently vendors the catch22 C source files directly, which means it can drift out of sync with upstream changes. This PR replaces that approach with a git submodule so pycatch22 stays synchronised with catch22.

To incorporate the C files from the catch22 submodule, the following changes were made:

Changes

• Pinned catch22 as a submodule at v0.4.1 (.gitmodules added)
• Moved the CPython wrapper (catch22_wrap.c) out of the old src/C/ into src/wrapper/, keeping it under pycatch22's own control
• Removed src/C/, which held the old vendored C code
• Updated setup.py to compile feature sources from src/catch22/C (excluding main) plus the wrapper, with include_dirs pointing at the submodule's C directory so bare #include "..." paths resolve
• Updated MANIFEST.in to exclude src/catch22/.git so the submodule's gitlink file isn't packed into sdists (the existing graft already bundles the upstream C files, keeping sdists self-contained)
• Updated .github/workflows/run_unit_tests.yaml to check out submodules recursively
• Updated README.md to document local installation via git clone --recursive-submodules
• Updated .github/workflows/run_unit_tests.yaml with python 3.12 and 3.13 platforms.
• bump version number to v0.5.0

The new file structure at pycatch22/src is:

pycatch22/src/
├─ catch22 @ pinned version/ # submodule → upstream catch22
├─ pycatch22/ # contains catch22_all
├─ wrapper/ # CPython wrapper

Note that this is a structural change only; no changes to computation or output have been made. catch22_all produces identical results to before, only the C source layout and build process have changed. All existing unit tests pass under the new submodule-based structure.