onnx-runtime-session

The user-facing session and inference API for the ORT 2.0 runtime (see docs/ORT2.md §20). Design goal: zero-config by default — the user never has to know what an execution provider is; the runtime auto-detects hardware and picks a strategy.

Phase 1 skeleton: the intent-based [SessionBuilder] and [InferenceSession] surfaces are defined; build/run bodies are todo!() pending the sequential executor (Phase 1 task ort2-session).

let mut session = onnx_runtime_session::load("model.onnx")?;
let outputs = session.run(&[("input_ids", &tensor)])?;