script-src 'self', object-src 'none' and base-uri 'self', enforced. The
two inline scripts the importmap helper emits are allowed by their
SHA-256, and the origins its URLs point to (a configured asset host, in
any form Rails accepts) are allowed by name. There is no per-request
nonce, so cached pages stay valid.
DISABLE_CONTENT_SECURITY_POLICY=true leaves the policy off, for an
install whose proxy or customization injects scripts.