docs: compare HEnRY with related approaches

[2mmanu]

Summary

• add a sourced research landscape covering interoperability, least privilege, identity and auditability
• compare architectural coverage without presenting every adjacent project as a direct competitor
• explain where HEnRY differs, where alternatives fit better and which production gaps remain open
• link the comparison from the README, problem statement and documentation map

Method

Focused review of primary publications, last reviewed 25 August 2026. Ratings describe published architectural scope and are not security certifications.

Validation

• PATH="$PWD/.venv/bin:$PATH" make check (13 tests)