Revise PEP to match implementation and improve writing

Align the PEP with the current reference implementation, resolve all
open issues, and rewrite for conciseness and natural tone.

Implementation alignment:
- Fix type description to int64_t, fix StopAsyncIteration name
- Document pickle format (2-tuple when disabled, 3-tuple when enabled)
- Document colorized output, control flow check uses C pointer identity
- Resolve all open issues into Rejected Ideas

Writing improvements:
- Cut from ~500 to ~300 lines by removing redundant subsections
- Rewrite Motivation: delete "Why Exception Groups Need Timestamps",
  move .add_note() comparison to Rejected Ideas, trim example analysis
- Convert numbered bold lists to prose throughout
- Add Maintenance Burden section with test suite impact data
- Remove non-ASCII characters and remaining LLM-isms
- Security Implications: None (opt-in, disabled by default)

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

Author: gpshead