Layout: remove the root ksp-fixtures directory; fixtures live inside the modules they test #108

Maintainer: a root ksp-fixtures/ was never approved. Move each consumer fixture under the module whose output it checks — e.g. Gradle-plugin consumer builds as Gradle TestKit functional-test fixture projects under python-multiplatform-gradle-plugin/src/functionalTest, KSP fixtures under python-multiplatform-ksp, the compose render/E2E fixtures under python-multiplatform-compose's tests — keeping every test and its separate-run property (AGENTS §15 updated). Completion: no root ksp-fixtures; the same test counts green per module; AGENTS/CI updated.