Add deterministic Python reference adapter

tests/fixtures/reference-python/src/sample/__init__.py

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+"""Small multi-file package for the Python reference adapter."""
+
+from .service import Service
+
+
+def package_name() -> str:
+    return Service.__name__

tests/fixtures/reference-python/src/sample/service.py

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+"""Service behavior with class, method, function, and local import evidence."""
+
+import json
+
+from .shared import clamp
+
+raise RuntimeError("The reference adapter must never execute project code")
+
+
+class Service:
+    def run(self, value: int) -> int:
+        return clamp(value) if json.__name__ else value
+
+
+async def build_service(enabled: bool) -> Service | None:
+    if enabled:
+        return Service()
+    return None

tests/fixtures/reference-python/src/sample/shared.py

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+"""Shared values used by dependent modules."""
+
+DEFAULT_LIMIT = 3
+
+
+def clamp(value: int, limit: int = DEFAULT_LIMIT) -> int:
+    if value < 0:
+        return 0
+    if value > limit:
+        return limit
+    return value

tests/fixtures/reference-python/src/sample/worker.py

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+"""A transitive dependent used to prove reverse invalidation."""
+
+from sample.service import Service
+
+
+def execute(value: int) -> int:
+    worker = Service()
+    return worker.run(value)