demo keys — for demonstration purposes only.

This directory ships pre-generated BGV keypairs (secret_key.bin + public_key.bin)
per key_id, so demo mode skips keygen. Layout: demo_keys/<key_id>/{secret_key,public_key}.bin

SECURITY: the secret keys here are PUBLIC (anyone with the package has them).
Data encrypted in demo mode is therefore NOT private. Never use demo mode for
real data. Regenerate with scripts/generate_demo_keys.py.
