test: gate V1 side files and V3 legacy receipts; early output rejection

- analyze_jet_cv_continuation: samples are analyzed only if the sibling
  re*_s*.json receipt is the same case with run_status COMPLETE and
  source_recheck UNCHANGED; receipts without run_status (pre-recheck
  runners) are refused unless --legacy-receipts, and labelled
  LEGACY_NO_FINAL_SOURCE_RECHECK. Results carry receipt_gate.
- V3: receipts are schema 2 and need source_recheck UNCHANGED; a missing
  recheck is accepted only for frozen-mode schema 1 receipts whose
  run_case.py hash is one of the pre-recheck runners (9805383, 2cf9853,
  db3d098; hashes checked against git history in a test).
- V1/V3 refuse --output inside the checkout before source checks or CUDA
  (shared source_provenance.reject_output_inside).
- A failing recheck is recorded as RECHECK_FAILED (still invalid); the
  in-run V1 receipt write is tmp+replace.
Negative host tests for each gate. No threshold, solver or evidence edits.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/validation/source_provenance.py

@@ -18,20 +18,28 @@ def checkout_provenance(repo):
 def final_source_check(recorded, recheck):
     """Re-derive the source identity after a run and compare it with the recorded one.
 
-    Returns receipt fields: source_recheck "UNCHANGED", or "CHANGED" with source_after
-    (None if the recheck itself failed) and the recheck error. Callers mark a receipt
-    complete only for "UNCHANGED"; a changed source invalidates the run.
+    Returns receipt fields: source_recheck "UNCHANGED"; "CHANGED" with source_after; or
+    "RECHECK_FAILED" (source_after None) with the error. Callers mark a receipt complete
+    only for "UNCHANGED"; anything else invalidates the run.
     """
     try:
         after = recheck()
-    except Exception as error:  # a failed recheck cannot confirm the source
-        return dict(source_recheck="CHANGED", source_after=None,
+    except Exception as error:  # a failed recheck cannot confirm the source: invalid too
+        return dict(source_recheck="RECHECK_FAILED", source_after=None,
                     source_recheck_error=f"{type(error).__name__}: {error}")
     if after == recorded:
         return dict(source_recheck="UNCHANGED")
     return dict(source_recheck="CHANGED", source_after=after, source_recheck_error=None)
 
 
+def reject_output_inside(output, repo):
+    """Refuse run outputs inside the checkout before any work: they would dirty it."""
+    from pathlib import Path
+    output, repo = Path(output).resolve(), Path(repo).resolve()
+    if output == repo or repo in output.parents:
+        raise SystemExit(f"--output must be outside the source checkout {repo}")
+
+
 class HistoryUnavailable(RuntimeError):
     """The clone cannot show the recorded commit (no git, shallow clone, installed copy)."""