Rewrite PEP for conciseness and natural tone

Cut from 501 to ~285 lines. Key changes:
- Trim Abstract and Motivation, delete redundant subsections
- Rewrite Rationale as focused prose
- Convert numbered lists to prose throughout
- Merge .add_note() discussion into Rejected Ideas
- Remove empty Open Issues section
- Tighten all Rejected Ideas entries
- Use proper reST footnote references

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gpshead