feat(audio): prefer dock/HDMI/USB sinks in WirePlumber (#87)

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.

modules/nixos/default.nix

@@ -125,6 +125,10 @@ in
       alsa.support32Bit = true;
       pulse.enable = true;
       wireplumber.enable = true;
+      # Prefer dock/HDMI/USB sinks when present; fall back to built-in when
+      # they disappear (BACKLOG #87). Rules: ./dock-audio-rules.nix.
+      wireplumber.extraConfig."90-nomarchy-dock-audio" =
+        import ./dock-audio-rules.nix;
     };
 
     # ── Desktop services ─────────────────────────────────────────────