Let `pytest-qt` own each real `PlotWidget` and input source so
fixture teardown closes them exactly once.
Also,
- select `PyQt6` and offscreen `Qt` before widget imports
- isolate `QSettings` and activate `pyproject.toml` config
- lock `pytest-qt` 4.5.0 and record verified harness commands
- retain real `tractor` dialog coverage beside `Qt` regressions
Prompt-IO: ai/prompt-io/opencode/20260831T005712Z_09ddcf50_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi