[DO NOT MERGE] Nested ATIF lineage POC for discussion

[AnuradhaKaruppiah]

Description

This PR shares a nested-lineage POC to align on how we represent and consume execution ancestry in ATIF, with emphasis on canonical tool_calls + Step.extra lineage mapping and upstreamability to AtifStep.

Docs to review in this PR

• Proposal: packages/nvidia_nat_eval/atif-rich-path-proposal.md
• Nested/branching eval example: examples/evaluation_and_profiling/simple_calculator_eval/src/nat_simple_calculator_eval/configs/config-branching-nested-trajectory-eval.yml
• End-to-end example README: examples/evaluation_and_profiling/simple_calculator_eval/README.md

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• When the PR is ready for review, new or existing tests cover these changes.
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[afourniernv]

LGTM