Use pipewire-gobject for PipeWire integration

[bhack]

Summary

• replace Mini EQ JACK/WirePlumber GI runtime integration with pipewire-gobject-backed PipeWire access
• use Pwg for registry discovery, metadata routing, node control params, module loading, and monitor capture
• update PyPI, Flatpak, CI, docs, and tests for pipewire-gobject>=0.3.1,<0.4

Validation

• .venv/bin/python -m ruff check .
• .venv/bin/python -m ruff format --check .
• PYTHONPATH=src .venv/bin/python tools/check_pipewire_gobject.py
• .venv/bin/python -m pytest -q
• .venv/bin/python -m build
• .venv/bin/python -m twine check dist/*
• fresh venv install of Mini EQ wheel using local pipewire-gobject 0.3.1 wheel; mini-eq --help and Pwg import passed

Dependency

• Depends on publishing pipewire-gobject 0.3.1 from Prepare 0.3.1 release pipewire-gobject#27 before this is ready to merge/release.

[bhack]

CI is being rerun now that pipewire-gobject 0.3.1 is published on PyPI. Keeping this PR draft-gated after CI: before merge or a Mini EQ release, we still need local app stress testing on this branch, including real music playback/routing/analyzer coverage.

[bhack]

Status update after pipewire-gobject 0.3.2:

• Updated Mini EQ to require pipewire-gobject>=0.3.2,<0.4.
• Updated the Flatpak manifest to build pipewire-gobject from tag 0.3.2 at commit 399f770d1f76847f6ae4d24ac856eff3d2214748.
• Adjusted CI to build the pipewire-gobject sdist in a plain build venv, then install that wheel into the system-site test/runtime venv so Ubuntu PyGObject packages remain available without breaking g-ir-scanner.
• PR checks are green, including test and Flatpak.

Keeping this PR as draft until real music stress testing on the app branch is complete.