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/capture/linux/mod.rs

@@ -1278,9 +1278,13 @@ mod pipewire {
         } else {
             None
         };
-        // PUNKTFUNK_FORCE_SHM=1 forces the race-free download path (SHM, no dmabuf) — required on
-        // Mutter+NVIDIA where dmabuf capture has no working sync and shows stale frames. KWin/
-        // gamescope don't need it (they blit into the buffer, so no read-before-render race).
+        // PUNKTFUNK_FORCE_SHM=1 forces the race-free download path (SHM, no dmabuf) — a manual
+        // escape hatch, mainly for Mutter+NVIDIA: that combo has no implicit dmabuf fence, so
+        // zero-copy capture can in principle race the compositor's render and show stale frames.
+        // Zero-copy is the Mutter+NVIDIA default (no unconditional override) since live retesting
+        // found no visible staleness; set this if you do see flashing/stale content on such a
+        // host. KWin/gamescope don't need it (they blit into the buffer, so no read-before-render
+        // race).
         let force_shm = std::env::var("PUNKTFUNK_FORCE_SHM").as_deref() == Ok("1");
         // VAAPI zero-copy passthrough: zero-copy on, no EGL→CUDA importer (any non-NVIDIA host), and
         // the encoder backend is VAAPI → hand the raw dmabuf to the encoder (it imports + GPU-CSCs).