feat(host): enable GPU zero-copy by default on all backends

Now that the per-capture worker subprocess makes an NVENC EGL/CUDA driver
fault survivable (design/zerocopy-worker-isolation.md), the reason the NVENC
zero-copy path stayed opt-in is gone. zerocopy::enabled() now defaults ON for
both GPU backends (was ON VAAPI / OFF NVENC). Fallbacks are intact: VAAPI's
one-shot CPU auto-downgrade (VAAPI-gated, never trips for NVENC) and NVENC's
per-capture fallback + worker-death latch.

Reframe the shipped host.env examples and setup guides to rely on the default
rather than force PUNKTFUNK_ZEROCOPY=1 (an explicit =1 skips the VAAPI
auto-downgrade).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/headless/run-headless-kde.sh

@@ -13,8 +13,8 @@
 #   bash scripts/headless/run-headless-kde.sh [WxH]    # default 1920x1080
 #
 # Then in another shell:
-#   WAYLAND_DISPLAY=wayland-kde XDG_CURRENT_DESKTOP=KDE PUNKTFUNK_ZEROCOPY=1 \
-#     punktfunk-host punktfunk1-host --source virtual --seconds 14400
+#   WAYLAND_DISPLAY=wayland-kde XDG_CURRENT_DESKTOP=KDE \
+#     punktfunk-host punktfunk1-host --source virtual --seconds 14400   # zero-copy is on by default
 set -euo pipefail
 
 RES="${1:-1920x1080}"

scripts/host.env.example

@@ -16,8 +16,9 @@ XDG_CURRENT_DESKTOP=KDE
 # resolution+refresh (the flagship mode); `portal` captures an existing monitor.
 PUNKTFUNK_VIDEO_SOURCE=virtual
 
-# GPU zero-copy capture (EGL/Vulkan → CUDA → NVENC). Falls back to CPU automatically.
-PUNKTFUNK_ZEROCOPY=1
+# GPU zero-copy capture (dmabuf → CUDA → NVENC / VAAPI / Vulkan) is ON by default and falls back to
+# CPU automatically. No need to set it. Set to 0 only to force the CPU path.
+# PUNKTFUNK_ZEROCOPY=0
 
 # --- Bazzite / SteamOS-like host: host-managed Steam-Deck-UI session -----------------------
 # The host LAUNCHES gamescope-session-plus headless AT THE CLIENT'S mode (so games see the