feat: MICE estimator selection via NonlinearityTag + Unpredictable all-fallback

[DEVunderdog]

Parent

#91

What to build

Replace the hardcoded BayesianRidge inside the MICE fitting block with adaptive estimator selection driven by NonlinearityTag. Collect the tag from each MICE column's NumericStats, take the most-complex tag across the block (ComplexNonlinear > MonotonicNonlinear > Linear > Unpredictable), and pass it with n_rows to RegressionEstimatorFactory. If every MICE column is Unpredictable, skip the MICE block entirely and fall each column back to Median via _fallback_to_median. Record the chosen estimator name and the driving NonlinearityTag in every MICE column's ColumnImputationRecord.signals.

This is the foundation slice — slices for dynamic max_iter/tol, initial_strategy, and n_nearest_features all build on the parameterised IterativeImputer construction introduced here.

Acceptance criteria

• MICE fitting block constructs IterativeImputer with the estimator returned by RegressionEstimatorFactory instead of the default BayesianRidge
• Most-complex-tag precedence is correctly applied: ComplexNonlinear > MonotonicNonlinear > Linear > Unpredictable
• When all MICE columns are Unpredictable: no MICE model is stored; each column's record is updated to Median strategy via _fallback_to_median with a clear signal
• Every MICE column's signals contains an entry recording the estimator name (e.g. "mice_estimator: GradientBoostingRegressor (tag=ComplexNonlinear)")
• Every MICE column's signals contains an entry recording the driving NonlinearityTag
• GradientBoostingRegressor is selected for ComplexNonlinear blocks above gradient_boost_min_rows; RandomForestRegressor for ComplexNonlinear blocks at or below that threshold
• Unit test: MICE block with at least one ComplexNonlinear column produces a non-linear estimator signal on all MICE column records
• Unit test: MICE block where all columns are Unpredictable produces no MICE model and each column's record shows Median strategy with a fallback signal
• Numpy-style docstrings are present on all in-scope symbols touched by this change

Blocked by

• Blocked by feat: add three MICE config fields to NumericImputationConfig #157