feat(vulkan): make Vulkan backends self-contained on the GPU (#10404)

Vulkan backends bundled their own loader and ICD manifests but neither the
Mesa driver the manifests point at nor a way to make the loader find them,
so on a runtime base image without Mesa the loader enumerated zero devices
and the GPU silently fell back to CPU (only NVIDIA worked, since its ICD is
injected by the container toolkit).

- scripts/build/package-gpu-libs.sh: for each installed ICD manifest, bundle
  the driver .so its library_path names — no hard-coded, platform-dependent
  soname list — plus that driver's ldd dependencies, skipping manifests whose
  driver isn't installed. Rewrite each library_path to a bare soname so the
  bundled driver resolves via the LD_LIBRARY_PATH run.sh already sets.
- .docker/install-base-deps.sh, backend/Dockerfile.golang,
  backend/Dockerfile.python: install mesa-vulkan-drivers in every Vulkan
  builder so the driver + manifests exist to be packaged (the LunarG SDK
  ships only the loader and shader tooling).
- pkg/model/process.go: when a backend ships vulkan/icd.d/, point the loader
  at it via VK_DRIVER_FILES/VK_ICD_FILENAMES at launch (no-op otherwise).
  Covered by pkg/model/process_vulkan_test.go.
- backend/go/parakeet-cpp/package.sh: complete the L0 stub (was missing the
  libc-family ldd walk + GPU-lib packaging) by mirroring whisper, so the
  vulkan-parakeet image actually bundles its GPU runtime.

Assisted-by: Claude Code:claude-opus-4-8

Signed-off-by: Richard Palethorpe <io@richiejp.com>

backend/Dockerfile.python

@@ -66,7 +66,12 @@ RUN <<EOT bash
             libwayland-dev libxrandr-dev libxcb-randr0-dev libxcb-ewmh-dev \
             git python-is-python3 bison libx11-xcb-dev liblz4-dev libzstd-dev \
             ocaml-core ninja-build pkg-config libxml2-dev wayland-protocols python3-jsonschema \
-            clang-format qtbase5-dev qt6-base-dev libxcb-glx0-dev sudo xz-utils
+            clang-format qtbase5-dev qt6-base-dev libxcb-glx0-dev sudo xz-utils && \
+        apt-get install -y mesa-vulkan-drivers libdrm2
+        # Mesa Vulkan ICD drivers (ANV/RADV/lavapipe) + their manifests. The
+        # LunarG SDK below only provides the loader and shader tooling, not
+        # hardware drivers — without Mesa, package-gpu-libs.sh has no ICD to
+        # bundle and the packaged backend finds no GPU at runtime.
         if [ "amd64" = "$TARGETARCH" ]; then
             wget "https://sdk.lunarg.com/sdk/download/1.4.335.0/linux/vulkansdk-linux-x86_64-1.4.335.0.tar.xz" && \
             tar -xf vulkansdk-linux-x86_64-1.4.335.0.tar.xz && \