Discussion: new plugin proposal pl-dicom_reorg for MRN/STUDY/SERIES normalization

# Discussion: new plugin proposal `pl-dicom_reorg` for MRN/STUDY/SERIES normalization

## Status
Discussion only. No implementation commitment in this ticket.

## Priority
Low (discussion)

## Labels
`discussion`, `low-priority`, `dicom`, `plugin`, `pl-dicom_reorg`, `data-layout`

## Context / Background
Input directories can be flat and include mixed Studies/Series. A deterministic pre-processing plugin could improve downstream push behavior and simplify troubleshooting.

## Detailed Scope (for discussion)
1. Create plugin `pl-dicom_reorg`.
2. Parse DICOM tags and organize files into:
- `MRN/StudyInstanceUID/SeriesInstanceUID/`
3. Define fallback policy for missing/invalid key tags.
4. Emit a manifest (`json` and/or `csv`) mapping source file to destination path and core identifiers.
5. Add dry-run mode.
6. Add tests for mixed input, malformed files, and missing tags.
7. Add docs and integration examples with `pl-dicom_push`.

## Acceptance Criteria (for implementation ticket)
- Mixed flat input is reorganized into deterministic hierarchy.
- Manifest output is complete and machine-readable.
- Malformed/non-DICOM files are reported explicitly.
- Docs include end-to-end usage pattern.

## Management Note
Owner: `Sandip`

Boundary:
- In scope: pre-push reorganization and manifesting.
- Out of scope: DICOM network transmission and PACS conformance handling.