Start Frametop's SteamVR clients only once SteamVR is up (#6)

The pointer and gaze services need steamvr.service to be running (Requisite=), and ft-pointer, ft-screens, and ft-gaze connect as a background app before switching to overlay, so they never start a vrserver of their own. One started from the dev container never finds the headset, which left a reboot stuck in a loop.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
(cherry picked from commit 51b4e79318)

pointer/helper/frametop-pointer.service

@@ -5,6 +5,7 @@ Documentation=file://@REPO@/README.md
 # Needs SteamVR's IPC; it starts and stops with SteamVR.
 After=steamvr.service frametop-input-relay.service
 PartOf=steamvr.service
+Requisite=steamvr.service
 
 [Service]
 # Runs in the dev container (built there against its libraries). The helper process