fix: make Guacamole fixture dual-stack and keep VM display awake

docs/implementation-plan.v1.md

@@ -893,6 +893,14 @@ watchdog/tunnel is bound
 only to the helper's private Docker gateway. Stop the helper before the normal
 `test-host reset`. It is a recovery interface, not a product component or a
 replacement for Guest Control/native test automation.
+Because Windows can power off the virtual monitor while the VM remains
+running, `test-host prepare` must also reapply the disposable display/sleep
+keepalive and record `prepare-display-keepalive`. A zero-bpp VRDE framebuffer
+otherwise leaves an otherwise authenticated Guacamole browser at “Waiting for
+response”. If the public browser hostname has an AAAA record, `rdp-access up
+--bind 0.0.0.0` must own a tracked IPv6-to-IPv4 forward and expose its
+`ipv6_forward` status; this prevents a browser selecting IPv6 for the WebSocket
+from silently taking a different, refused path.
 
 For this provisioned lane, the approved host-only credential-file location is
 `/home/cabbage/.local/share/rvbox-secrets/rvbox-win10-test.password`. It must