expressions: no test validates dispatch routes to concrete implementations, not stubs

[discreteds]

Problem

The expressions protocol alignment tests (test_protocol_alignment.py) verify that every protocol method has a concrete implementation on the composed ExpressionSystem class. But no test validates that the __getattr__ dispatch mechanism on BaseExpressionAPI actually routes to a real implementation rather than an inherited Protocol stub.

Gap

Consider a scenario where:

1. A builder class inherits SomeProtocol (which declares method_x)
2. The builder forgets to implement method_x
3. The alignment test checks the composed ExpressionSystem — but __getattr__ dispatch operates on individual builder classes

The alignment test passes (because another builder or the composed class might have method_x), but the dispatch routes to the wrong builder's inherited stub.

What's needed

A test that:

1. For each method discoverable via _FLAT_NAMESPACES dispatch, verifies the dispatched method is a concrete implementation (defined in __dict__ of the builder class), not an inherited Protocol stub
2. Alternatively: a test that instantiates the API, calls each method name through normal attribute access, and verifies it doesn't return None/... (the protocol stub return values)

Relationship to #114

Issue #114 proposes fixing the dispatch to use ns_cls.__dict__ instead of hasattr. That fix makes this test less critical (stubs would never be dispatched), but the test remains valuable as defence-in-depth — verifying that every method the dispatch could route is actually implemented.

Discovered during

Codex adversarial review of the Relation API builder decomposition spec (2026-05-06).