Run-tree correlation for telemetry spans.

Wraps :telemetry.span/3 so that every span carries a :run_id and a :parent_run_id in its metadata. Nested spans (graph invoke → step → node → LLM call / checkpoint) form a tree that can be reconstructed from the emitted events, which is the foundation for tracing UIs and the OpenTelemetry bridge.

The current run id lives in the process dictionary. Node tasks run in separate processes, so the executor captures the parent's run id before spawning and seeds it with inherit_run_id/1 inside the task.