fix(evidence): re-check the tree once more before recording a result

The last tree check ran after the final mutation's restore; the result was
built a few statements later. Check again immediately before the result
exists, so the recorded snapshot and the tree it describes cannot drift in
that window. The recorded run is unaffected: its definition hash and its
commit are unchanged.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016vuEiNoEXaooMDRu6tXrZ6

Author: PhysShell

scripts/mutate_campaign.py

@@ -523,6 +523,7 @@ def restore() -> None:
         with open(os.path.join(ROOT, t), encoding="utf-8") as f:
             if f.read() != text:
                 raise CampaignError(f"{t} was not restored to its pristine content")
+    assert_tree_unchanged(baseline, "before recording the result")
     return Result(
         campaign=definition.campaign,
         definition_sha256=definition.sha256,