feat: skew-driven initial_strategy for MICE IterativeImputer

[DEVunderdog]

Parent

#91

What to build

Set initial_strategy on the MICE IterativeImputer based on the skew profile of the MICE block rather than always defaulting to "mean". If any MICE column has SkewSeverity >= Moderate, use "median"; otherwise use "mean". Record the decision in every MICE column's ColumnImputationRecord.signals.

Acceptance criteria

• IterativeImputer for the MICE block is constructed with initial_strategy set to "median" when any MICE column has SkewSeverity >= Moderate
• IterativeImputer is constructed with initial_strategy="mean" when all MICE columns have SkewSeverity < Moderate (or no skew stats available)
• Every MICE column's signals contains an entry recording the chosen initial_strategy and the reason (e.g. "mice_initial_strategy: median (skewed column detected)")
• Unit test: MICE block with at least one Moderate-or-higher skew column produces initial_strategy=median in all MICE column signals
• Unit test: MICE block with all normal-skew columns produces initial_strategy=mean in signals

Blocked by

• Blocked by feat: MICE estimator selection via NonlinearityTag + Unpredictable all-fallback #158