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.

flake.nix

@@ -510,6 +510,61 @@
             '';
           };
 
+          # Dock audio sink priority (#87): WirePlumber rules prefer
+          # HDMI/DP/USB sinks over built-in analog. Pure unit test of the
+          # rule set + a minimal VM that the conf file is installed where
+          # WirePlumber reads it (XDG_DATA_DIRS / share/wireplumber).
+          dock-audio =
+            let
+              inherit (nixpkgs) lib;
+              rules = import ./modules/nixos/dock-audio-rules.nix;
+              alsa = rules."monitor.alsa.rules" or [ ];
+              expect = cond: msg: lib.assertMsg cond "dock-audio: ${msg}";
+              json = builtins.toJSON alsa;
+              pureOk =
+                expect (builtins.isList alsa && builtins.length alsa >= 2)
+                  "expected ≥2 alsa rule groups"
+                && expect (lib.hasInfix "hdmi" json)
+                  "HDMI match missing from rules JSON"
+                && expect (lib.hasInfix "usb-" json || lib.hasInfix "usb" json)
+                  "USB match missing from rules JSON"
+                && expect (lib.hasInfix "priority.session" json)
+                  "priority.session missing"
+                && expect (lib.hasInfix "1100" json)
+                  "HDMI session priority 1100 missing";
+            in
+            assert pureOk;
+            pkgs.testers.runNixOSTest {
+              name = "nomarchy-dock-audio";
+              nodes.machine = { ... }: {
+                services.pipewire = {
+                  enable = true;
+                  alsa.enable = true;
+                  pulse.enable = true;
+                  wireplumber.enable = true;
+                  wireplumber.extraConfig."90-nomarchy-dock-audio" =
+                    import ./modules/nixos/dock-audio-rules.nix;
+                };
+                # WirePlumber is a user service; enable linger-less check via
+                # the system data dir the package puts conf into.
+                environment.systemPackages = [ pkgs.wireplumber ];
+              };
+              testScript = ''
+                machine.wait_for_unit("multi-user.target")
+                conf = machine.succeed(
+                    "find /nix/store -path "
+                    "'*wireplumber.conf.d/90-nomarchy-dock-audio.conf' "
+                    "2>/dev/null | head -1"
+                ).strip()
+                assert conf, "90-nomarchy-dock-audio.conf not in the store"
+                body = machine.succeed(f"cat '{conf}'")
+                assert "monitor.alsa.rules" in body, body
+                assert "priority.session" in body, body
+                assert "hdmi" in body.lower() or "HDMI" in body, body
+                assert "usb" in body.lower(), body
+              '';
+            };
+
           # Clamshell / dock lid policy (#86): logind must ignore lid close
           # when docked (external display present). Real multi-monitor lid
           # events stay V3; this asserts the baked logind.conf + that