fp-stability: harden log preservation and report writing (review feedback)

- _preserve_logs runs in _run_case's finally block; an OSError there
  (disk full, dest collision) would replace the case's real outcome and,
  being uncatchable by the suite's MFCException handler, abort the whole
  run. Make it best-effort with a printed warning, matching the adjacent
  ignore_errors rmtree.
- _emit_github_summary creates log_dir if missing instead of raising
  FileNotFoundError at the very end of a run.

Author: sbryngelson

toolchain/mfc/fp_stability.py

@@ -533,7 +533,12 @@ def _run_case(
                 cons.print(f"  [bold yellow]float-max check error[/bold yellow]: {exc}")
 
     finally:
-        _preserve_logs(work_dir, os.path.join(log_dir, name))
+        # best-effort, like the rmtree below: a failed log copy must not replace
+        # the case's real outcome (this runs even when the try block is raising)
+        try:
+            _preserve_logs(work_dir, os.path.join(log_dir, name))
+        except OSError as exc:
+            cons.print(f"  [bold yellow]could not preserve logs[/bold yellow]: {exc}")
         shutil.rmtree(work_dir, ignore_errors=True)
         cons.unindent()
         cons.print()

toolchain/mfc/fp_stability_report.py

@@ -168,6 +168,7 @@ def _emit_github_summary(results: list, n_samples: int, log_dir: str = None):
 
     text = "\n".join(md) + "\n"
     if log_dir:
+        os.makedirs(log_dir, exist_ok=True)
         with open(os.path.join(log_dir, "summary.md"), "w") as f:
             f.write(text)
     summary_path = os.environ.get("GITHUB_STEP_SUMMARY")