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>

Cargo.lock

@@ -121,6 +121,15 @@ dependencies = [
  "rustversion",
 ]
 
+[[package]]
+name = "ash"
+version = "0.38.0+1.3.281"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0bb44936d800fea8f016d7f2311c6a4f97aebd5dc86f09906139ec848cf3a46f"
+dependencies = [
+ "libloading",
+]
+
 [[package]]
 name = "ashpd"
 version = "0.13.11"
@@ -1486,6 +1495,7 @@ dependencies = [
  "aes",
  "aes-gcm",
  "anyhow",
+ "ash",
  "ashpd",
  "axum",
  "axum-server",