feat(evidence): evaluate retrieval with a small fixture

harness/tht/evidence/preprocessing.py

@@ -1,5 +1,6 @@
 """Explicit construction boundary for Evidence preprocessing."""
 
+from collections.abc import Callable
 from typing import Protocol
 
 from tht.evidence.contracts import EvidenceSource
@@ -26,6 +27,7 @@ def build_preprocessing_pipeline(
     retain_published_generations: int = 3,
     workspace_id: str | None = None,
     sparse_language: str = "italian",
+    candidate_evaluator: Callable[[object], object] | None = None,
 ) -> CorpusPipeline:
     """Construct preprocessing from the bounded infrastructure supplied by core."""
     return CorpusPipeline(
@@ -40,6 +42,7 @@ def build_preprocessing_pipeline(
         retain_published_generations=retain_published_generations,
         workspace_id=workspace_id,
         sparse_language=sparse_language,
+        candidate_evaluator=candidate_evaluator,
     )