Prepare an unsigned CREATE ENVIRONMENT transaction for wallet signing.

Absorbs check-token + check-authority + get-blockhash + tx building into a single call. The frontend signs the returned transaction and sends it via /send-transaction.

Requires a pending registration (call /start first).

==== Observability ==== Every decision branch in this function emits a structured log line via the log binding (see below). Each log event includes every value that affected the decision, so grepping kubectl logs | grep register_prepare for a given token_address or user_wallet gives the complete picture of what the user saw without having to ask them.