fix(host+vdisplay): a mode-switch replacement display inherits the group topology

The resize rebuild creates the new virtual display BEFORE retiring the old
one (create-before-drop), so the registry saw a live same-backend sibling —
its own dying predecessor — and told the backend it was not first in group.
Mutter then skipped the Primary/Exclusive apply ("joining an existing
display group — extending") and the retiring owner took the topology with
it: every resize silently demoted the virtual output to an extended,
shell-less desktop. Thread the superseded pool gen through acquire so the
replacement establishes topology, and stop counting kept (Lingering/Pinned)
entries as demoting siblings — no session owns them, so there is no live
desktop to clobber.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/gamestream/stream.rs

@@ -404,6 +404,7 @@ fn open_gs_virtual_source(
         // close code — wiring it to skip-linger is a follow-up, so this path keeps normal keep-alive
         // (a fresh, never-set flag).
         std::sync::Arc::new(std::sync::atomic::AtomicBool::new(false)),
+        None, // fresh session — no display superseded
     )
     .context("create virtual output at client resolution")?;
     // HDR: pass the negotiated `cfg.hdr` (client asked for HDR AND the host can deliver it). On the