Resolve Ruff safety findings in the PyQt task model

[ZFordDev]

Closes #180

This PR replaces call expressions in Qt model defaults with a shared invalid QModelIndex, explicitly marks the sort map as class-owned state, and adds behavior coverage for root and child model indexes.

This is the final PR in the Ruff remediation stack and is stacked on #183.

Validation:

• Ruff check passes across src and tests with zero findings.
• Ruff formatting passes for all 79 files.
• The complete source and test trees compile.
• The PyQt behavior test is committed for GitHub Actions; PyQt6 is not installed in the local bundled runtime.
• Local mypy is unavailable in the bundled runtime, so CI will run it after installing declared dev dependencies.