feat: n_nearest_features selection for large MICE blocks via Pearson correlation

[DEVunderdog]

Parent

#91

What to build

For large MICE blocks, compute n_nearest_features from value-level Pearson correlations already available in the CorrelationProfiler output, rather than leaving it unset (which forces IterativeImputer to use all predictors). For small blocks, leave n_nearest_features=None.

Algorithm:

• If the MICE block has ≤ mice_n_nearest_features_min_cols columns: set n_nearest_features=None (use all predictors)
• Otherwise: for each MICE column, count how many other MICE columns have |Pearson r| > mice_correlation_threshold (from CorrelationProfiler). Take the median count across all MICE columns, cap at mice_max_nearest_features, and pass the result as n_nearest_features
• Record the decision (and the computed value, or "all predictors") in every MICE column's signals

All three threshold fields (mice_n_nearest_features_min_cols, mice_max_nearest_features, mice_correlation_threshold) come from NumericImputationConfig (added in #157).

Acceptance criteria

• MICE blocks larger than mice_n_nearest_features_min_cols columns have n_nearest_features computed from CorrelationProfiler value-level Pearson correlations
• MICE blocks at or below mice_n_nearest_features_min_cols have n_nearest_features=None
• Computed n_nearest_features is capped at mice_max_nearest_features
• Only columns with |Pearson r| > mice_correlation_threshold count as informative predictors
• Every MICE column's signals records the n_nearest_features decision (value used, or "all predictors used — block below min_cols threshold")
• Unit test: MICE block exceeding mice_n_nearest_features_min_cols produces a n_nearest_features signal entry on all MICE column records
• Unit test: MICE block at or below the threshold produces an "all predictors" signal and n_nearest_features=None is passed to IterativeImputer
• Numpy-style docstrings on any new helper functions

Blocked by

• Blocked by feat: add three MICE config fields to NumericImputationConfig #157
• Blocked by feat: MICE estimator selection via NonlinearityTag + Unpredictable all-fallback #158