docs(readme): tighten headline + eval table, fix demo link

- Collapse the multi-thousand-char "Headline metrics" walls into a
  scannable summary; full per-version audit trail now points to
  docs/03_eval_methodology.md and docs/SESSION_HANDOFF.md
- Collapse the 40-row eval table to milestone checkpoints + pointer
- Fix broken demo-video link (raw/master -> raw/main; no master branch)
- Drop stale 92.0%/81.0% screenshot/video captions; state current 94.0%

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: JuliaEdom