docs: add external validation research framework

[floppy522]

Summary

• add a pre-declared external-validation research plan for H1-H4
• add a problem-first interview guide and zero-result discovery template
• add controlled-pilot methodology and a zero-result results register
• add research-to-product decision traceability

Evidence boundary

This PR adds methodology and templates only. It does not claim that interviews, workflow observations, pilots, external adoption, or human time savings have occurred. Raw participant materials remain outside the public repository.

Verification

• Markdown lint: 6 files, 0 issues (MD013 disabled for tables)
• relative links resolved
• zero-result guards verified
• focused demo seeder tests: 52 passed
• independent documentation review: no Critical, Important, or Minor findings

Docker is unavailable in the local work environment, so Compose smoke/E2E are delegated to the existing PR CI.