Fix ROCm backend (target MIGraphX EP) and add CUDA support

ROCm silently fell back to CPU: the code targeted ONNX Runtime's classic
ROCMExecutionProvider, but distro ROCm-enabled ONNX Runtime builds (e.g.
Arch's onnxruntime-rocm) are commonly compiled with --use_migraphx instead,
and registration failures were invisible since breadmill never installed a
tracing subscriber. Switches the rocm feature to target MIGraphX, adds a
default tracing subscriber so EP registration success/failure is always
visible, and fixes a real crash where MIGraphX's output sequence padding
could index the attention mask out of bounds during mean-pooling.

Also adds a CUDA backend (--cuda / backend = "cuda") mirroring the same
ort execution-provider pattern, for NVIDIA hardware.

Version bump: 0.1.0 -> 0.2.0.

breadsearch-shared/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "breadsearch-shared"
-version = "0.1.0"
+version = "0.2.0"
 edition = "2021"
 license = "MIT"