feat: isolate simultaneous RDP fixture profiles

docs/testing-vm.md

@@ -94,6 +94,10 @@ down/reset lifecycle; it is not an alternative to the native test controller.
 In public-bind mode the helper also owns a tracked IPv6-to-IPv4 `socat` forward
 when the browser hostname has an AAAA record; check `ipv6_forward=active` in
 `rdp-access status` before diagnosing a browser-side “Waiting for response”.
+When more than one already-running VM needs browser access, set matching
+`RVBOX_TEST_VBOX_*`/`RDP_ACCESS_VRDE_PORT` values and a unique
+`RDP_ACCESS_PROFILE` plus HTTP/tunnel ports; the helper README has the
+copy/paste example and explains the native-host lease boundary.
 
 ## Snapshots and reset contract