Assert the generator output against a golden fixture and forbid logic in it

Problem/Motivation

Two risks of this work are known: the generator silently changing the API, and logic leaking into generated files, which would turn every site running it into a generated fork. Both are testable, and neither is caught by testing the responses alone.

Proposed resolution

• Golden files: generate from the fixture model and compare byte for byte with a committed expected output, so any change to a template is visible in review.
• A test that parses every generated class and fails on a conditional, a loop or a service call in it — the rule of "declarations only" enforced mechanically.
• A test that the generated code passes the project's PHPCS and PHPStan.
• A test that generating twice produces no diff.

Remaining tasks

Everything.