feat: track last_changed_field for time-travel navigation

Add last_changed_field property to ObjectState that tracks the field
that most recently changed VALUE (not just dirty status).

Key difference from last_dirty_field:
- last_dirty_field: tracks dirty status changes (clean->dirty or dirty->clean)
- last_changed_field: tracks actual value changes regardless of saved state

This is essential for time-travel navigation because:
- When undoing and a field goes from dirty->clean, we still want to scroll to it
- Shows what changed in the transition, not just current dirty state

Stores _last_changed_paths from _recompute_fields() and exposes via property.

Author: trissim

src/objectstate/object_state.py

@@ -2418,8 +2418,24 @@ def _recompute_invalid_fields(self) -> Set[str]:
                     # Update provenance for this field
                     self._live_provenance[dotted_path] = (source_scope_id, source_type)
 
+        # Store for navigation - fields that changed value in this computation
+        self._last_changed_paths = changed_paths
+        if changed_paths:
+            self._last_changed_field = sorted(
+                changed_paths,
+                key=lambda field: (field == "func", -field.count("."), field),
+            )[0]
         return changed_paths
 
+    @property
+    def last_changed_field(self) -> Optional[str]:
+        """Field that most recently changed value (not just dirty status).
+        
+        This tracks any value change regardless of saved/unsaved state,
+        useful for time-travel navigation to show what changed in a transition.
+        """
+        return getattr(self, "_last_changed_field", None)
+
     def reset_parameter(self, param_name: str) -> None:
         """Reset parameter to signature default (None for lazy dataclasses).