fix(evidence): bind corpus to workspace identity

harness/tht/search/evidence.py

@@ -3,12 +3,17 @@
 from tht.corpus.store import CorpusStore
 
 
+class CorpusWorkspaceMismatchError(RuntimeError):
+    """The configured workspace does not own the persisted corpus."""
+
+
 class ActiveEvidenceSearcher:
     """Searcher facade that enforces ACTIVE generation predicates before LIMIT."""
 
-    def __init__(self, corpus: CorpusStore, delegate):
+    def __init__(self, corpus: CorpusStore, delegate, expected_workspace_id: str | None = None):
         self.corpus = corpus
         self.delegate = delegate
+        self.expected_workspace_id = expected_workspace_id
 
     def search(self, embedding, top_n=10, kinds=None, metadata_filter=None):
         requested = set(kinds) if kinds is not None else {
@@ -31,6 +36,12 @@ class ActiveEvidenceSearcher:
             if include_evidence:
                 manifest = self.corpus.active_manifest()
                 workspace_id = manifest.metadata.get("workspace_id") if manifest else None
+                if manifest is not None and self.expected_workspace_id is not None and (
+                    workspace_id != self.expected_workspace_id
+                ):
+                    raise CorpusWorkspaceMismatchError(
+                        "corpus belongs to a different workspace; use a new corpus root or rebuild"
+                    )
                 if manifest is not None and isinstance(workspace_id, str):
                     by_generation: dict[str, list[str]] = {}
                     mapping = dict(manifest.metadata.get("document_generations", {}))
@@ -50,9 +61,9 @@ class ActiveEvidenceSearcher:
         return sorted(hits, key=lambda hit: (-hit.similarity, hit.id))[:top_n]
 
 
-def active_searcher(cfg, delegate):
+def active_searcher(cfg, delegate, *, workspace_id: str | None = None):
     corpus_root = cfg.paths.artifacts.parent / "corpus"
-    return ActiveEvidenceSearcher(CorpusStore(corpus_root), delegate)
+    return ActiveEvidenceSearcher(CorpusStore(corpus_root), delegate, workspace_id)
 
 
 def resolve_evidence_file(