feat: M2 zero-copy foundation — EGL→CUDA import + NVENC CUDA-frame path

Scaffolding for dmabuf zero-copy (plan §9), opt-in via LUMEN_ZEROCOPY:

- src/zerocopy/{cuda,egl}.rs: hand-rolled CUDA Driver-API FFI (no Rust crate
  exposes the EGL-interop calls / CUeglFrame) with a shared process-wide
  CUcontext + pitched device buffers; an EGL importer (GBM platform on the
  NVIDIA render node) that turns a dmabuf into an EGLImage, registers it with
  CUDA, and copies it device-to-device into an owned buffer. `zerocopy-probe`
  subcommand validates the FFI/linking/GPU access — confirmed on the box
  (driver 595, EGL_EXT_image_dma_buf_import + modifiers).
- CapturedFrame gains a FramePayload enum (Cpu(Vec<u8>) | Cuda(DeviceBuffer));
  the encoder branches: CPU keeps the expand+upload path, CUDA wraps the device
  buffer in an AV_PIX_FMT_CUDA frame fed straight to hevc_nvenc (sharing our
  CUcontext via a hand-declared AVCUDADeviceContext, since ffmpeg-sys doesn't
  bind hwcontext_cuda.h). open_video/the encoder take a `cuda` flag derived from
  the first frame's payload.

The capture-side dmabuf negotiation (which produces the Cuda frames) is the
next step; the CPU path is unchanged and remains the default + fallback. Builds
clean, clippy clean, tests pass.

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

crates/lumen-host/src/main.rs

@@ -23,6 +23,8 @@ mod pipeline;
 mod pwinit;
 mod vdisplay;
 mod web;
+#[cfg(target_os = "linux")]
+mod zerocopy;
 
 use anyhow::{bail, Result};
 use encode::Codec;
@@ -52,6 +54,9 @@ fn real_main() -> Result<()> {
         // Standalone input-injection smoke test (no client needed): open the session's input
         // backend and inject a scripted mouse/keyboard pattern. Watch a focused app / `wev`.
         Some("input-test") => input_test(),
+        // Zero-copy FFI/GPU probe: init the EGL importer + CUDA context (no capture needed).
+        #[cfg(target_os = "linux")]
+        Some("zerocopy-probe") => zerocopy::probe(),
         // M0 pipeline spike.
         Some("m0") => m0::run(parse_m0(&args[1..])?),
         Some("-h") | Some("--help") | Some("help") | None => {