docs: clarify README install surface

[Zer0pa-Architect-Prime]

Summary

• add the marked INSTALL-DX README install block
• preserve the existing front-door table and authority sections byte-for-byte from the first table onward

Verification

• install_dx_readme.py second pass --check: unchanged
• git diff --check
• first onward unchanged vs origin/main
• README contains INSTALL-DX marked block
• no package installs, venv creation, or node_modules creation