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>=0.3.3,<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 --expect-version 0.3.3
• .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 the published pipewire-gobject 0.3.3 sdist; mini-eq --help and Pwg import passed
• local app stress testing with real music is in progress and remains the draft gate before merge/release

Dependency

• pipewire-gobject 0.3.3 is published on PyPI/TestPyPI and pinned in the Flatpak manifest from tag 0.3.3 at commit d7be87c57fae342443fe532b525e00bdfac83102.

[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.

[bhack]

Status after pipewire-gobject 0.3.3 release:

• PR body updated to reference pipewire-gobject>=0.3.3,<0.4 and the published 0.3.3 tag.
• Added a small GI cleanup commit declaring the GLibUnix namespace version before import.
• Local validation passed:
  • .venv/bin/python -m ruff format --check .
  • .venv/bin/python -m ruff check .
  • .venv/bin/python -m pytest
  • tools/run_release_preflight_container.sh
• Container preflight installed pipewire-gobject 0.3.3 from PyPI, built Mini EQ artifacts, ran twine checks, verified --check-deps, and confirmed the sibling Flathub manifest drift check passes.

Keeping the PR draft-gated for the remaining real-session checks: local Flatpak runtime smoke, clean-permission background portal smoke, and interactive real-music testing before merge/release.