Fix failing checks

[kendonB]

Title

Fix failing CI checks: Qt/PySide compatibility, platform deps, ubuntu-latest runner

Description

This PR improves cross-platform compatibility (especially macOS/Linux CI) by:

• Adding a small Qt compatibility layer (castervoice/lib/qt.py) to support both PySide2 and PySide6 (enum/access differences + exec vs exec_).
• Updating Qt UI modules to use that wrapper:
  • castervoice/asynch/hud.py
  • castervoice/asynch/settingswindow.py
  • castervoice/asynch/hmc/h_launch.py
  • castervoice/asynch/hmc/homunculus.py
• Making natlink optional in castervoice/lib/ctrl/mgr/engine_manager.py so imports/mode changes don’t fail on non-Windows systems.
• Hardening/fixing edge cases in castervoice/rules/core/text_manipulation_rules/text_manipulation_support.py (argument validation + cursor/selection offset calculations, including TeXstudio paths).
• Updating requirements-mac-linux.txt to use platform markers for dragonfly2 extras and Qt bindings.
• Switching the Linux GitHub Actions runner to ubuntu-latest in .github/workflows/testrunner.yml.

Motivation and Context

CI environments (and non-Windows installs) were failing

How Has This Been Tested

• python3 -m compileall on the modified modules (syntax check).
• GitHub Actions workflow completed here.
• Cursor BugBot check
• Codex Review Agent check

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue or bug)

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have checked that my code does not duplicate functionality elsewhere in Caster.
• My code implements all the features I wish to merge in this pull request.
• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests pass.

Maintainer/Reviewer Checklist

• Basic functionality has been tested and works as claimed.
• New documentation is clear and complete.
• Code is clear and readable.