feat(host): KWin virtual output primary + settle portal env on switch

Two parked follow-ups from the session-aware host work:

#3 — KWin/Mutter virtual output not set primary. The auto-detected desktop path
*is* "stream this desktop", but the per-session virtual output wasn't promoted to
primary, so KDE/GNOME panels + windows stayed on an unstreamed real output and the
streamed screen showed only wallpaper. apply_session_env now defaults
PUNKTFUNK_KWIN_VIRTUAL_PRIMARY / PUNKTFUNK_MUTTER_VIRTUAL_PRIMARY on for the
auto path (explicit config still wins), so the streamed output becomes the sole
desktop.

#2 — input flaky after a mid-stream Gaming->Desktop switch. The xdg portal
(D-Bus-activated) and the systemd --user env still pointed at the old session, so
the host's RemoteDesktop portal opened against a half-stale env: it accepted
events but they didn't reach the compositor until a reconnect. New
vdisplay::settle_desktop_portal() pushes the live session env into the
systemd/D-Bus activation environment and (for KWin) restarts the portal so it
re-reads it, mirroring a fresh desktop login (and the existing wlroots portal
restart). Called from the mid-stream switch rebuild slot before the injector
reopens. GNOME uses Mutter's direct EIS, so it only gets the env push.

Compiles, clippy/fmt clean, 78 host tests pass. Live validation on the Bazzite
box next.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/m3.rs

@@ -2014,6 +2014,15 @@ fn virtual_stream(
                     env: sw.env,
                 });
                 crate::vdisplay::apply_input_env(sw.compositor);
+                // Switching INTO a desktop mid-stream: the xdg portal / systemd-user env may still
+                // point at the old session, so input would silently not land until a reconnect.
+                // Settle it (env push + KWin portal restart) before the injector reopens against it.
+                if matches!(
+                    sw.compositor,
+                    crate::vdisplay::Compositor::Kwin | crate::vdisplay::Compositor::Mutter
+                ) {
+                    crate::vdisplay::settle_desktop_portal(sw.compositor);
+                }
                 // Build the new backend's pipeline BEFORE dropping the old one (retry absorbs the
                 // brief compositor-coexistence race during a switch); on failure keep the old.
                 let rebuilt =