Harden live UI stress checks

[bhack]

Summary

• expand the AT-SPI widget smoke to verify GtkDropDown option exposure through combo-box expand/collapse semantics
• extend the live UI runtime smoke to probe pipewire-gobject, route a synthetic stream, follow default-output metadata changes, and verify filter output retargeting
• wait for WirePlumber default-output metadata before writing configured defaults in the private PipeWire fixture

Validation

• .venv/bin/python tools/check_pipewire_gobject.py --expect-version 0.3.4
• .venv/bin/python -m ruff check tools/check_live_ui_runtime.py tests/test_mini_eq_atspi_widgets.py
• .venv/bin/python -m py_compile tools/check_live_ui_runtime.py
• MINI_EQ_RUN_ATSPI=1 .venv/bin/python -m pytest tests/test_mini_eq_atspi_widgets.py -q
• .venv/bin/python tools/check_live_ui_runtime.py --timeout 45 --cycles 2 --audio-duration 120
• MINI_EQ_RUN_LIVE_UI=1 .venv/bin/python -m pytest tests/test_mini_eq_live_ui_runtime.py -q

Manual real-music listening remains a release gate, not a merge gate for this PR.