Main motivation for this is spyder, which depends on qtwebengine.
Part of #480196

Testing methodology:

• nixpkgs-review to get rebuild list
• execute programs to make sure they start, both with QT_QPA_PLATFORM=wayland and QT_QPA_PLATFORM=xcb
• nix why-depends to ensure no qt5 leaks into qt6 apps
• Fix whatever was broken

Programs Tested

• magicgui: dependency to napari, choses QT backend automatically now
• napari: Works on python 3.12 and 3.13

• python3.14Packages.napari: broken because of napari-npe2, not Qt
• angr-management: runs
• inkcut: Updated to qt6, runs

• napari-console: dependency for napari
• qtawesome: dependency for spyder
• qtconsole: dependency for spyder
• superqt: dependency for most of these, choses QT backend automatically now
• enamix: dependency to inkcut
• rare: Still runs, but update to Qt6 likely needs an update of the software and i do not have EpicGames to test
• sasview: already was qt6, runs
• typstwriter: already was qt6, runs
• spyder: Does not find kernels unless run with nix-shell -I nixpkgs=. -p "(python3.withPackages (python-pkgs: [python-pkgs.spyder]))", but this issue exists on master already. I do not believe this is a blocker.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.