Add Qt6 OBS builds for RPM and Arch distributions

[pbek]

Summary

Extend the desktop-cmake3 Open Build Service package beyond Debian-family builds by adding Qt6/CMake package definitions for RPM-based distributions and Arch Linux.

Changes

• Add a Qt6 CMake RPM spec for Fedora and openSUSE
• Add a Qt6 CMake PKGBUILD for Arch Linux
• Publish those build descriptions through build-for-obs.sh
• Add a helper that enables only OBS repositories with the required Qt6 development packages
• Keep build, publish, debuginfo, and use-for-build repository settings aligned
• Add python314Packages.osc to the devenv

Initial OBS targets

• Fedora 36 through 44 and Rawhide
• openSUSE Tumbleweed
• openSUSE Leap 16.0
• Arch Linux
• Existing supported Debian, Ubuntu, and Raspbian Qt6 targets

openSUSE Slowroll is excluded because its repository currently does not provide cmake(Qt6WebSockets). Older targets without the required Qt6 development packages remain disabled.

Validation

• just nix-build
• shellcheck on the OBS scripts
• bash -n on the OBS scripts
• git diff --check
• RPM targets have been scheduled successfully by OBS after uploading the spec file.

[pbek]

26.9.2

• Added Qt6 Open Build Service configurations for Fedora, openSUSE, and Arch Linux
  (for #3727)