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.

docs/ROADMAP.md

@@ -394,6 +394,12 @@ Design/decision records and a running log of shipped work (items marked
   power management is on (systemd docked = external display present).
   Undocked lid suspend left to upstream. `checks.clamshell-logind` asserts
   logind.conf + active logind. Real lid+external V3 in HARDWARE-QUEUE.
+  ✓ **Dock audio sink priority (#87, `VISION § C`):** WirePlumber
+  `90-nomarchy-dock-audio` raises `priority.session` for HDMI/DP and USB
+  ALSA sinks so they become default when present; unplug falls back to
+  built-in. Rules in `modules/nixos/dock-audio-rules.nix`;
+  `checks.dock-audio` pure + conf-file VM. Real hotplug V3 in
+  HARDWARE-QUEUE.
 - ✓ **Hibernate double-unlock:** on resume from hibernate the LUKS
   passphrase already gates the machine, but locking hyprlock before sleep
   meant a second password. Fixed by *not locking* before an encrypted