Support independently controlled Wave devices and capture keepalives

[rikkichy]

Scope

• Enumerate exact USB units, including multiple devices of the same model.
• Serialize each unit's transfers and firmware synchronization separately; preserve healthy handles during hotplug.
• Bind queued controls to their selected device, cancel delayed sliders on selection changes, and finish workers before disconnecting.
• Maintain one capture keepalive per Wave input; preserve pins when PipeWire observation fails.

Verification

• 26 focused Python tests pass.
• Nix package builds.
• Isolated GTK/Xvfb scenario passes: two emulated units, serial selector, write ownership, stale drag cancellation, hotplug survivor, clean shutdown. Actual physical multi-device hardware was not exercised.

Draft while dynamic routing integration is verified. No change to unsupported 00c7 USB variant.