test: verify get_total_key_supply is scoped per creator (issue #701)

Adds 17 unit tests covering the full acceptance criteria for issue #701:
supply isolation per creator, cross-creator non-interference on both
buys and sells, monotonic increment/decrement, multi-buyer aggregation,
three-creator independence, sell-to-zero, idempotent read behaviour, and
NotRegistered enforcement on the checked supply view.

Closes #701

Author: stableprogrammer