feat: M2 — Vulkan bridge: TRUE zero-copy for gamescope's LINEAR dmabufs (Phase 3)

The missing zero-copy path is closed. NVIDIA's EGL won't sample LINEAR and the CUDA driver
rejects raw dmabuf fds — but Vulkan imports dmabufs (VK_EXT_external_memory_dma_buf) and
exports OPAQUE_FD memory that CUDA officially imports. zerocopy/vulkan.rs (ash):

  dmabuf fd → VkBuffer (import cached per fd) → vkCmdCopyBuffer (GPU) →
  exportable VkBuffer → vkGetMemoryFdKHR(OPAQUE_FD) → cuImportExternalMemory → CUdeviceptr

The exportable buffer + CUDA mapping are per-resolution; per frame it's one GPU buffer copy
(fence-waited) + one pitched CUDA copy into the encoder's pool. No CPU touches pixels.
EglImporter::import_linear now routes through the bridge (lazy init; any failure still falls
back to the CPU mmap path). cuda::ExternalDmabuf gained import_owned_fd for the
Vulkan-exported fd.

Validated live: gamescope 720p120 → "Vulkan→CUDA exportable staging buffer ready
size=3686400" (exactly 1280*720*4), full-rate 122.7 fps, decoded frame pixel-correct
(vkcube). KWin's tiled EGL path regression-tested intact. NV12 negotiation dropped — moot
now that BGRx is fully zero-copy.

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

crates/lumen-host/Cargo.toml

@@ -70,3 +70,7 @@ futures-util = "0.3"
 # eglCreateImage + the dma_buf import; the CUDA driver API (EGL interop) and libgbm are linked
 # via hand-rolled FFI in `src/zerocopy/` (no Rust crate exposes the EGL-interop driver calls).
 khronos-egl = { version = "6", features = ["dynamic"] }
+# Vulkan bridge for LINEAR dmabufs (gamescope): import via VK_EXT_external_memory_dma_buf,
+# GPU-copy into an exportable allocation, export OPAQUE_FD → cuImportExternalMemory (the
+# officially-supported CUDA pairing; raw dmabuf fds are rejected by the desktop driver).
+ash = "0.38"