Add ETL as-is analysis report

[GVCUTV]

Motivation

• Provide a comprehensive, code-based analysis of the /etl Python pipeline so maintainers and simulation engineers have a single, authoritative reference of responsibilities, I/O, data contracts, execution ordering, and known risks.

Description

• Add docs/etl/ETL_AS_IS_ANALYSIS.md which exhaustively documents every *.py under /etl (inventory, per-file high-level analysis, data contracts map, execution graph, cross-cutting concerns, risks, and open questions) based strictly on repository code inspection.

Testing

• No automated tests were run because this is a documentation-only change; the new file was created and committed to the repository.

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Codex Task