Add OpenVINO backend for Intel iGPU/dGPU (Arc)

Mirrors the rocm/cuda pattern: ort::ep::OpenVINO with device_type "GPU"
(covers both Intel integrated graphics and Arc discrete GPUs through the
same EP), load-dynamic/dlopen so no OpenVINO toolkit is needed at build
time, and its own --openvino flag / backend = "openvino" config value.
Folded into the `full` feature alongside npu/rocm/cuda.

OpenVINO's provider options go through a generic key/value FFI interface
rather than a fixed C struct (unlike MIGraphX's OrtMIGraphXProviderOptions),
so it should be less exposed to the ABI-version-skew crash MIGraphX hit --
but that's inference from the EP's design, not verified against real
hardware. Like CUDA, this is compile-checked only: no Intel GPU in this
dev environment to runtime-verify against.

Version bump: breadmill 0.2.2 -> 0.2.3.

breadsearch-shared/src/lib.rs

@@ -129,7 +129,8 @@ pub struct ModelConfig {
     pub name: String,
     #[serde(default = "default_dim")]
     pub dim: usize,
-    /// Compute backend: "cpu", "npu" (VitisAI/XDNA), "rocm" (MIGraphX/AMD GPU), or "cuda" (NVIDIA GPU).
+    /// Compute backend: "cpu", "npu" (VitisAI/XDNA), "rocm" (MIGraphX/AMD GPU),
+    /// "cuda" (NVIDIA GPU), or "openvino" (Intel iGPU/dGPU).
     #[serde(default = "default_backend")]
     pub backend: String,
 }