fix(evidence): enforce active vector generations

harness/tht/corpus/store.py

@@ -7,6 +7,7 @@ import fcntl
 import os
 import re
 import stat
+import shutil
 import uuid
 from pathlib import Path
 from contextlib import contextmanager
@@ -137,6 +138,13 @@ class CorpusStore:
             raise UnsafeCorpusPath("manifest must not be a symlink")
         return CorpusManifest.model_validate_json(manifest_path.read_text(encoding="utf-8"))
 
+    def discard(self, generation: str) -> None:
+        path = self.generation_path(generation)
+        if path.exists():
+            if path.is_symlink() or not stat.S_ISDIR(path.lstat().st_mode):
+                raise UnsafeCorpusPath("generation cleanup target is unsafe")
+            shutil.rmtree(path)
+
     def active_manifest(self) -> CorpusManifest | None:
         generation = self.active_generation()
         return self.manifest(generation) if generation else None