feat: M2 — complete zero-copy dmabuf→NVENC capture path (EGL/GL→CUDA)

The PipeWire dmabuf now reaches NVENC with no CPU touch. Verified live against
headless KWin: a tiled BGRx dmabuf is imported and encoded to a pixel-correct
H.265 stream (decoded frame matches the captured desktop — no tiling artifacts,
no colour swap). The CPU-copy path stays the default and the runtime fallback.

Capture side (zerocopy::egl): desktop NVIDIA can't register a dmabuf EGLImage
with CUDA directly (cuGraphicsEGLRegisterImage is Tegra-only; cuGraphicsGLRegisterImage
rejects EGLImage-backed textures), so we follow OBS/Sunshine — bind the EGLImage
to a GL texture, render it through a fullscreen-triangle shader into an immutable
GL_RGBA8 texture (de-tiling + .bgra swizzle to the BGRx the encoder wants), then
register that texture with CUDA and copy it device-to-device into an owned buffer
so the dmabuf returns to the compositor immediately.

Encode side (encode/linux::submit_cuda): take a *pooled* CUDA surface via
av_hwframe_get_buffer and device→device-copy our imported buffer into it, instead
of wrapping our own pointer in a bare AVFrame. A bare frame is rejected with
EINVAL (NVENC ignores frames with null buf[0]; the encode path's av_frame_ref
needs a refcounted buffer), and a fresh device pointer every frame would thrash
NVENC's bounded resource-registration cache — the pool recycles a small set.

Also: gate FFmpeg AV_LOG_DEBUG behind LUMEN_FFMPEG_DEBUG for diagnosing
hw-frame rejects, and refresh the now-accurate module docs.

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

crates/lumen-host/src/capture/linux.rs

@@ -547,7 +547,7 @@ mod pipewire {
                 // Zero-copy path: if the buffer is a dmabuf and we have an importer, import it
                 // into a CUDA device buffer (no CPU touch) and deliver that. Otherwise fall
                 // through to the shm de-pad copy below.
-                if let (Some(importer), Some(fmt)) = (ud.importer.as_ref(), ud.format) {
+                if let (Some(importer), Some(fmt)) = (ud.importer.as_mut(), ud.format) {
                     if datas[0].type_() == pw::spa::buffer::DataType::DmaBuf {
                         let plane = crate::zerocopy::DmabufPlane {
                             fd: datas[0].fd(),