Add missing Pack.has_NaNs() method for restart tests

The restart roundtrip tests call pack.has_NaNs() to detect NaN
output, but the method was never implemented on the Pack class.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sbryngelson

toolchain/mfc/packer/pack.py

@@ -40,6 +40,12 @@ def find(self, filepath: str) -> PackEntry:
     def set(self, entry: PackEntry):
         self.entries[entry.filepath] = entry
 
+    def has_NaNs(self) -> bool:
+        """Return True if any entry contains NaN values."""
+        import math
+
+        return any(math.isnan(d) for e in self.entries.values() for d in e.doubles)
+
     def remove(self, filepath_or_entry: typing.Union[str, PackEntry]):
         if isinstance(filepath_or_entry, str):
             del self.entries[filepath_or_entry]