opencv-python-headless: add build-opencv-python-headless.yml for riscv64 wheels

[luhenry]

Adds .github/workflows/build-opencv-python-headless.yml, building the headless OpenCV 5.0.0.93 Python bindings for riscv64.

Shape — the headless distribution is not a separate project: it is the same opencv/opencv-python tree built with ENABLE_HEADLESS=1, which makes setup.py name the package opencv_python_headless and append -DWITH_QT=OFF -DWITH_GTK=OFF -DWITH_MSMF=OFF -DWITH_OBSENSOR=OFF -DOPENCV_FFMPEG_ENABLE_LIBAVDEVICE=OFF. The workflow therefore mirrors the merged build-opencv-python.yml (#390) rather than inventing a second recipe: build-from-checkout with cibuildwheel, one cp37-abi3 wheel (upstream always configures -DPYTHON3_LIMITED_API=ON and builds --py-limited-api=cp37, injected here via CIBW_CONFIG_SETTINGS) built on cp312 and reused to test 3.13 and 3.14. Upstream publishes no free-threaded wheel, so there is no cp314t entry.

The one non-obvious bit — setup.py calls get_and_set_info(), which regenerates cv2/version.py only when .git exists and otherwise reads back headless from that file, overwriting whatever ENABLE_HEADLESS said. Since the workflow removes the git repositories before the build (to keep ~1 GB out of the container), the flag has to be set in both places: find_version.py False True False False at the stamp step, asserted with grep -Fqx "headless = True", and ENABLE_HEADLESS=1 in CIBW_ENVIRONMENT.

Version mapping — unchanged from #390: upstream tags a release by its build number alone (5.0.0.93 is tag 93), the plain PyPI version stays the workflow input so the nightly PyPI check works, and the tag is derived from it.

riscv64 fix — CMAKE_ARGS=-DPNG_RISCV_RVV=off, same as #390: OpenCV's bundled libpng defaults PNG_RISCV_RVV to CPU_RVV_SUPPORTED, which OpenCV sets from a -march=rv64gc_v dispatch probe; libpng then re-probes the intrinsics with the baseline flags, fails, and message(FATAL_ERROR ...).

Divergence from upstream — upstream builds inside quay.io/opencv-ci/opencv-python-manylinux*, which is not published for riscv64; CI_BUILD is left unset. For the headless build that costs nothing that headless would have kept — the CI_BUILD branch that links Qt5 and bundles DejaVu fonts is itself gated on not build_headless.

Testing mirrors scripts/install.sh: tests/get_build_info.py, then OpenCV's own Python suite (modules/python/test/test.py -v --repo .) with OPENCV_TEST_DATA_PATH pointing at an opencv_extra checkout at the pinned submodule commit.

CI is starting from the pull_request trigger; the sibling non-headless build takes ~2 h per run.

CI — run 32965920143 is green: one opencv_python_headless-5.0.0.93-cp37-abi3-manylinux_2_39_riscv64.whl (16.4 MB) built on cp312 in ~2 h and reused for cp313/cp314 (Found previously built wheel ... Skipping build step). abi3audit --strict reports cv2.abi3.so is_abi3: true, baseline 3.7. Configure logs GUI: NONE with -DWITH_QT=OFF -DWITH_GTK=OFF -DWITH_MSMF=OFF -DWITH_OBSENSOR=OFF -DOPENCV_FFMPEG_ENABLE_LIBAVDEVICE=OFF, i.e. the headless flags did reach cmake. OpenCV's Python suite ran on all three interpreters: Ran 155 tests ... OK (skipped=10) each time. publish dry-ran cleanly — Dry run (not on main branch — no upload will happen) / Would upload 1 file(s).