Raise priority.session for HDMI/DisplayPort (1100) and USB (1050) ALSA
outputs so the default sink follows dock plug/unplug; built-in analog
wins again when externals disappear. Rules live in
modules/nixos/dock-audio-rules.nix; checks.dock-audio covers the contract.
Verified: V2 — flake check --no-build; checks.dock-audio green.
V3 pending: HARDWARE-QUEUE #87 real HDMI/USB-dock hotplug.
