docs(research): add cleanroom rewrite research artifacts

Three upstream research artifacts plus an index README for the May 2026
cleanroom rewrite of reactiflux.com:

- 2026-05-15-reactiflux-audience-research.md — 5-section grounding brief
- 2026-05-17-reactiflux-audience-taxonomy.md — 10 identity profiles
- 2026-05-17-reactiflux-site-structure.md — site architecture and Perspective Bar
- README.md — reading order, reconciliation notes, open questions

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vcarl