Support OpenCV 5; cover both majors in CI

[kmatzen]

Fixes #56.

OpenCV's config treats major versions as incompatible, so find_package(OpenCV 4.0 REQUIRED) rejected 5.x outright. That is what broke macOS CI when Homebrew moved the opencv formula to 5.0.0. #57 papered over it by pinning to the keg-only opencv@4, which unblocked CI but left the underlying question open.

I checked before changing anything

Rather than relax the constraint and hope, I ran a throwaway probe job (run 29665231151) that built against OpenCV 5.0.0 on macOS with only the version pin changed:

-- Found OpenCV: /opt/homebrew/Cellar/opencv/5.0.0 (found version "5.0.0")
1/1 Test #1: framewright_tests ................   Passed    1.67 sec
100% tests passed out of 1

Library, examples and the full suite all clean. That pass is real, not the empty-suite result from #55 — ctest runs with --no-tests=error now.

This matches expectations: the OpenCV surface here is small (cv::Mat, cv::Vec3b, CV_8UC3) and none of it changed in 5.x.

Change

The constraint is unversioned with an explicit floor:

find_package(OpenCV REQUIRED)
if(OpenCV_VERSION VERSION_LESS 4.0)
    message(FATAL_ERROR "framewright requires OpenCV 4.0 or newer, found ${OpenCV_VERSION}")
endif()

A genuinely too-old OpenCV still fails, now with a readable message instead of the config-file rejection. The resolved version is printed at configure time.

CI covers both majors

Rather than trade one silent breakage for another. Pinning only to opencv@4 would let a future 5.x incompatibility land unnoticed; testing only 5 would drop the version most distributions still ship — Ubuntu's libopencv-dev is still 4.x, so the Linux job continues to cover that path too.

Matrix is now Linux (4.x via apt), macOS + opencv@4, macOS + opencv (5.x).

🤖 Generated with Claude Code