Fix flake8 F821 false positive for forward reference in base.py

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

Author: brycewang-stanford

src/statspai/core/base.py

@@ -18,7 +18,7 @@ def __init__(self):
         self._results = None
 
     @abstractmethod
-    def fit(self, **kwargs) -> 'EconometricResults':
+    def fit(self, **kwargs) -> 'EconometricResults':  # noqa: F821
         """
         Fit the econometric model