EP bridge: ORT plugins called by onnx-light

onnx_light_ort::bridge_ep_ort_called_by_light is currently an interface target depending on Shared support for hosting ORT-compatible plugins. It will host foreign EP factory/device callbacks, provider selection and kernel-registry or compiled-partition execution in onnx-light.

EP-specific registration belongs here. API tables, opaque values, tensor ownership and resource lifetime utilities are shared with Custom-op bridge: ORT custom ops called by onnx-light through the common library, never by linking the two bridges to each other. This direction must not install, load or link ORT.

Runtime implementation and executable tests remain planned. The first implementation must add factory/device ownership, partial failure, capability selection, kernel/compiled-state reuse and teardown tests using independent fixture EPs. Common-layer dependency smoke tests do not validate EP hosting.