Prepare 0.8.9: reduce idle DSP and analyzer work

[bhack]

Summary

• Reduce unused DSP processing while preserving all 32 logical band slots and mute/solo/bypass behavior.
• Reduce hidden and frozen analyzer work and coalesce spectrum UI updates.
• Keep the playback device stable during preset/filter changes and restore playback on shutdown.
• Include the required PipeWire loopback module in Flatpak packaging and add regression coverage.
• Prepare 0.8.9 release metadata.

Validation

• 654 tests passed; 2 opt-in tests skipped. Lint, formatting, and release preflight passed.

• Native audio/routing checks passed at 48 and 192 kHz, including sparse-band and all-Off configurations.

• Flatpak dependency/routing checks and GTK/AT-SPI runtime smoke passed.

• Automated MPRIS regression checks and real playback testing verified uninterrupted playback during preset changes and shutdown.

• Fader and graph timing checks passed within the existing performance budgets.

• Clean-permission background access, start-at-login configuration, hidden-window lifecycle, and D-Bus show/quit checks passed in a real GNOME session.