Module schema preflight: the discovered-module path and the Update delta path have no tests

Two paths in the #535 schema preflight are covered only by reading the code:

1. A discovered third-party module gets the check at load time. Every test sets `Discover = false`, so the load-time path never runs under test.
2. An Update delta on a module's own existing table, the `Owner.Owns` path with a difference that is not a Create, has no test.

One test each. Behaviour looks right; the gate that proves it does not exist.

Found while verifying #535.