design: machine-readable validate output with store/reload

Design plan for enhancing `dandi validate` with:
- Structured output formats (-f json/json_pp/json_lines/yaml)
- Auto-save _validation.jsonl sidecar alongside .log files
- --load to reload/re-render stored results with different groupings
- Upload validation persistence for later inspection
- Extended grouping options (severity, id, validator, standard, dandiset)
- Refactoring into dandi/validate/ subpackage (git mv separately)
- _record_version field on ValidationResult for forward compatibility
- VisiData integration via native JSONL support

Addresses #1515, #1753, #1748; enhances #1743.

Co-Authored-By: Claude Code 2.1.63 / Claude Opus 4.6 <[email protected]>

Author: yarikoptic