fix(evidence): resolve final review findings

harness/tht/cli/preprocess_cmd.py

@@ -45,8 +45,19 @@ def _evaluation_workspace_root(cfg) -> Path:
     return root
 
 
+def _requires_candidate_evaluation(cfg) -> bool:
+    evidence = cfg.evidence
+    if evidence is None or evidence.schema_version != 2:
+        return False
+    return evidence.source_root is not None or any(
+        source.type == "filesystem" for source in evidence.sources
+    )
+
+
 def _candidate_evaluator(cfg, *, vector_store, embedder):
     """Bind candidate publication to the same read-only retrieval evaluator as the CLI."""
+    if not _requires_candidate_evaluation(cfg):
+        return None
     from tht.evidence.canonical import load_curated_tree
     from tht.evidence.evaluation import evaluate_retrieval, load_evaluation_fixture
 
@@ -77,9 +88,7 @@ def _candidate_evaluator(cfg, *, vector_store, embedder):
 
 def _validate_materialized_curated_corpus(cfg) -> None:
     """Fail closed on a v2 pinned filesystem corpus before any vector write is possible."""
-    if cfg.evidence is None or cfg.evidence.schema_version != 2:
-        return
-    if not any(source.type == "filesystem" for source in cfg.evidence.sources):
+    if not _requires_candidate_evaluation(cfg):
         return
     from tht.evidence import validate_workspace_evidence