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.

README.md

@@ -278,7 +278,7 @@ option, e.g. `services.easyeffects.enable = lib.mkForce false;` — see
 | `nomarchy.system.fileManager.enable` | `true` | Thunar GUI + gvfs/tumbler/udisks2 (the "open folder" handler) |
 | `nomarchy.system.greeter.enable` | `true` | greetd/tuigreet |
 | `nomarchy.system.greeter.autoLogin` | `null` | Auto-login this user into Hyprland (installer sets it on LUKS machines) |
-| `nomarchy.system.audio.enable` | `true` | Pipewire stack |
+| `nomarchy.system.audio.enable` | `true` | Pipewire stack; WirePlumber prefers HDMI/USB (dock) sinks when present so default output follows dock/undock |
 | `nomarchy.system.bluetooth.enable` | `true` | Bluetooth + blueman |
 | `nomarchy.system.autoTimezone.enable` | `false` | Automatic timezone (geoclue + automatic-timezoned) — the clock follows your location; toggle from System › Auto timezone (a menu enable rebuilds: it has to unset the static `time.timeZone`) |
 | `nomarchy.system.snapper.enable` | `false` | Hourly/daily BTRFS timeline snapshots + `nixos-rebuild-snap` (installer enables it; no-op unless root is BTRFS) |