Reorganize PyPNM collection and op-id flow

Feature: align PyPNM paths and MultiCapture operation IDs
- Move DS OFDM getCapture endpoints under PNM/Downstream/OFDM
- Add Start test scripts to capture operation_id per MultiCapture flow
- Store per-flow IDs and keep op_id fallback for compatibility
- Update Results/Status/Stop URLs to use per-flow operation ID vars
- Update analysis bodies to use per-flow operation_id variables
- Use visualizer header path mapping so sync remains stable after reorg - 2026-02-28 22:45:24

Author: mgarcia01752