feat(host/capture): zero-copy by default on VAAPI hosts (dmabuf passthrough)

PUNKTFUNK_ZEROCOPY unset now defaults ON when the encode backend is
VAAPI — a stock AMD/Intel install gets the LINEAR-dmabuf -> GPU-CSC path
instead of three full-frame CPU touches (measured on the 780M at 1440p:
0.8s vs 7.9s CPU per 600 frames, pixel-identical). NVENC stays opt-in.
A dmabuf offer the compositor never accepts latches a one-shot downgrade
so the pipeline rebuild renegotiates on the CPU offer; explicit =1 keeps
erroring loudly. The EGL->CUDA importer is no longer built on VAAPI
backends (an NVIDIA box forced to PUNKTFUNK_ENCODER=vaapi now correctly
takes the passthrough instead of producing CUDA frames the encoder
rejects), and a VAAPI session landing on the CPU path warns with the
reason.

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

CLAUDE.md

@@ -452,7 +452,10 @@ Pinned crate facts: `ashpd` 0.13 + `pipewire` 0.9 (must match ashpd's) + `ffmpeg
 (`ffmpeg-sys-next` auto-detects the system FFmpeg, so it builds against **FFmpeg 7.x/libavcodec 61
 or 8.x/libavcodec 62** — validated live on Ubuntu 26.04 (8) and Bazzite F43 (7.1); the zero-copy
 FFI also link-needs `libGL`/`libgbm`/`libcuda` at build time). Env knobs: `PUNKTFUNK_VIDEO_SOURCE=virtual|portal`,
-`PUNKTFUNK_COMPOSITOR=kwin|gamescope|mutter`, `PUNKTFUNK_ZEROCOPY=1`, `PUNKTFUNK_GAMESCOPE_APP=...`,
+`PUNKTFUNK_COMPOSITOR=kwin|gamescope|mutter`, `PUNKTFUNK_ZEROCOPY=1|0` (Linux default: ON for
+VAAPI/AMD/Intel with a one-shot CPU downgrade if the dmabuf offer never negotiates, OFF/opt-in for
+NVENC), `PUNKTFUNK_VAAPI_LOW_POWER=1|0` (pin the VAAPI entrypoint; auto = full-feature then VDEnc
+fallback for modern Intel), `PUNKTFUNK_GAMESCOPE_APP=...`,
 `PUNKTFUNK_INPUT_BACKEND=...`, `PUNKTFUNK_PERF=1` (per-stage timing), `PUNKTFUNK_VIDEO_DROP=N` (FEC
 test), `PUNKTFUNK_FEC_PCT=N`, `PUNKTFUNK_DSCP=1` (opt-in DSCP/SO_PRIORITY media QoS on the data +
 GameStream video/audio sockets; no-op on the wire on Windows without a qWAVE policy),