Docs: Add comprehensive data verification system documentation

Document the multi-layer verification architecture, all 7 failure modes
encountered (with dates, evidence, root causes, and fixes), the golden
rules distilled from failures, and why the system is effective.

Tells the complete story of ensuring data integrity in an LLM-driven
statistical publication.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dshkol