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 ─────────────────────────────────────────────

modules/nixos/dock-audio-rules.nix

@@ -0,0 +1,38 @@
+# WirePlumber 0.5 ALSA rules: prefer dock / external sinks over built-in
+# analog when they appear (BACKLOG #87). Higher priority.session wins as
+# the default node; when HDMI/USB goes away, the next-highest (usually
+# the laptop speakers/headphones) becomes default again.
+#
+# Pure attrset so checks.dock-audio can unit-test the contract without a
+# full PipeWire stack. Consumed by modules/nixos/default.nix.
+#
+# Priorities stay ≤1500 (WirePlumber docs: sinks default ~600–1000; going
+# much higher can outrank streams / BT in surprising ways).
+{
+  "monitor.alsa.rules" = [
+    {
+      # HDMI / DisplayPort monitor audio (names vary: .hdmi-stereo,
+      # .hdmi-surround, HiFi__HDMI1__sink, …).
+      matches = [
+        { "node.name" = "~alsa_output\\..*\\.hdmi-.*"; }
+        { "node.name" = "~alsa_output\\..*HDMI.*"; }
+        { "node.name" = "~alsa_output\\..*\\.DisplayPort.*"; }
+        { "node.name" = "~alsa_output\\..*\\.dp-.*"; }
+      ];
+      actions."update-props" = {
+        "priority.driver" = 1100;
+        "priority.session" = 1100;
+      };
+    }
+    {
+      # USB dock / dongle audio (Thunderbolt dock, USB-C audio, …).
+      matches = [
+        { "node.name" = "~alsa_output\\.usb-.*"; }
+      ];
+      actions."update-props" = {
+        "priority.driver" = 1050;
+        "priority.session" = 1050;
+      };
+    }
+  ];
+}