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.

agent/BACKLOG.md

@@ -55,10 +55,6 @@ runtime-computed `resume_offset` into the boot cmdline, and the dev box runs
 exactly this layout (`/proc/cmdline` verified) but isn't Nomarchy. Steps in
 HARDWARE-QUEUE. Close #76 on PASS.
 
-### 87. Dock audio sink follows plug/unplug (`VISION § C`)
-Default audio sink: dock sink on plug, internal on unplug (WirePlumber
-policy or udev hook). Complements #86; can ship independently.
-
 ## LATER
 
 - **Wallpapers artifact split** (ROADMAP § Faster switches — decided,
@@ -111,8 +107,8 @@ high-ROI, etc.) live in the journal + ROADMAP — not here.*
   2026-07-10). Keep #59 commented install guidance; no
   `nomarchy.hardware.nvidia.*` until a hybrid maintainer + queue.
 
-_(#80–#83 + #85–#86 shipped 2026-07-11. Remaining dock slice: **#87**
-  dock audio. #86 V3 clamshell on real external display → HARDWARE-QUEUE.)_
+_(#80–#83 + #85–#87 shipped 2026-07-11. Dock V3 residuals: #86
+  clamshell + #87 audio on real hardware → HARDWARE-QUEUE.)_
 
 ### v1.0 pointer