fix(evidence): enforce active vector generations

harness/tht/corpus/pipeline.py

@@ -72,6 +72,8 @@ class CorpusPipeline:
             return self._run(dry_run=dry_run, resume=resume)
 
     def _run(self, *, dry_run: bool = False, resume: str | None = None) -> PipelineResult:
+        generation = None
+        vector_written = False
         previous = self.store.active_manifest()
         try:
             discovered = self._discover()
@@ -138,6 +140,7 @@ class CorpusPipeline:
             records = [self._vector_record(part, vector, generation) for part, vector in zip(changed_chunks, embeddings, strict=True)]
             if records:
                 written = self.vector_store.upsert("evidence", records)
+                vector_written = True
                 if written != len(records):
                     raise PipelineError("vector write count mismatch")
             generation_path = self.store.generation_path(generation)
@@ -156,11 +159,26 @@ class CorpusPipeline:
                 )
             self.store.publish(staged)
         except PipelineError:
+            self._compensate(generation, vector_written)
             raise
         except Exception as error:
+            self._compensate(generation, vector_written)
             raise PipelineError("Evidence preprocessing failed") from error
         return PipelineResult("succeeded", generation, True, changed, unchanged, removed, self.store.manifest(generation))
 
+    def _compensate(self, generation: str | None, vector_written: bool) -> None:
+        if generation is None:
+            return
+        try:
+            self.store.discard(generation)
+        except Exception:
+            pass
+        if vector_written:
+            try:
+                self.vector_store.delete_generation("evidence", generation)
+            except Exception:
+                pass
+
     @staticmethod
     def _vector_record(chunk: CanonicalChunk, embedding: list[float], generation: str):
         record = VectorRecord(