Orin is headless: drop local video out, ALSA instead of PipeWire

The tablet app is the display, so the diagram's hud (local video out)
branch goes; displays are all remote. The car unit now runs no desktop
and no sound server, so the host layer lists ALSA, and mic capture for
Whisper reads it directly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -19,9 +19,10 @@ rest down.
    CAN         ──▶ can-bridge ──┐
    OBD-II      ──▶ obd-poller ─┤
    GNSS        ──▶ gnss        ├──▶  message bus  ──┬──▶ recorder (disk)
-   camera      ──▶ vision*     ─┘                   ├──▶ hud (local video out)
+   camera      ──▶ vision*     ─┘                   └──▶ telemetry-gw ──▶ remote displays (tablet app)
-                                                    └──▶ telemetry-gw ──▶ remote displays
                           * later phase, separate task
+
+The Orin runs headless: no local video out. The tablet app is the display.
 ```
 
 ## Services

docs/ORIN-RUNTIME.md

@@ -20,7 +20,7 @@ repo, so it lives in the hub rather than in any one of them.
   acquisition    can-bridge, obd-poller, gnss
   bus            mosquitto
   ─────────────────────────────────────────────────────────
-  host           kernel modules, gpsd, PipeWire, network     orin-nano-env-setup
+  host           kernel modules, gpsd, ALSA, network         orin-nano-env-setup
 ```
 
 A layer may depend on the one below it and on the bus. Nothing depends sideways:
@@ -180,6 +180,10 @@ Whisper runs on the Orin, so its input does too. Audio *in* on one box and audio
 *out* on the other is asymmetric on purpose: each side does the simple thing, and
 a microphone has no reason to reach the head unit.
 
+The Orin runs headless with no sound server: capture reads the ALSA device
+directly. If a service ever needs PipeWire, run it for that service's user
+(lingering), not as a desktop session.
+
 ### Degrade when there is no sink
 
 With no tablet connected the Orin can still hear — the mic is local — but cannot