fix(host): stop force-disabling zero-copy on Mutter+NVIDIA by default

apply_session_env unconditionally forced PUNKTFUNK_FORCE_SHM=1 for every
GNOME/Mutter session, added 2026-06-14 after a same-day stale-frame bug hunt
found Mutter has no implicit dmabuf fence on NVIDIA. That override silently
contradicted the documented "zero-copy is on by default for all Linux GPU
backends" behavior and left Mutter+NVIDIA hosts on the slower CPU/SHM path
unconditionally, with no way to opt back in.

Live retesting (192.168.1.21, RTX 5070 Ti, real client with cursor
movement/window drag/typing — the historical trigger) shows no visible
staleness with the override removed. Drop the automatic force; PUNKTFUNK_FORCE_SHM
stays as a manual escape hatch for anyone who does hit flashing/stale frames
on a Mutter+NVIDIA host.

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

crates/punktfunk-host/src/vdisplay.rs

@@ -619,11 +619,6 @@ pub fn apply_session_env(active: &ActiveSession) {
     if let Some(d) = &e.xdg_current_desktop {
         std::env::set_var("XDG_CURRENT_DESKTOP", d);
     }
-    // Mutter on NVIDIA has no working dmabuf capture sync — force SHM there; the KWin/gamescope
-    // tiled/LINEAR paths keep zero-copy.
-    if active.kind == ActiveKind::DesktopGnome {
-        std::env::set_var("PUNKTFUNK_FORCE_SHM", "1");
-    }
     // Topology (Stage 2): the per-compositor backends (KWin/Mutter) now read
     // [`effective_topology`] directly at create time — the console policy, else the legacy
     // `PUNKTFUNK_{KWIN,MUTTER}_VIRTUAL_PRIMARY` env, else the Auto default (exclusive on the