feat(imputation): add ImputationStrategy.MNAR enum member and remove mnar_constant_fill from NumericImputationConfig

[DEVunderdog]

Parent

#97

What to build

Add MNAR = "mnar" to the ImputationStrategy enum and remove the mnar_constant_fill: float = -1 field from NumericImputationConfig entirely — including its to_dict() and from_dict() entries. Constant = "constant" may remain in the enum as a dead value so the backward-compat migration shim (a later slice) can still read it, but it must not be used anywhere in new code.

This slice establishes the vocabulary that all other Scope 8 slices build on.

Acceptance criteria

• ImputationStrategy.MNAR exists with serialised value "mnar".
• NumericImputationConfig no longer declares mnar_constant_fill; instantiating it with mnar_constant_fill=<any> raises TypeError.
• NumericImputationConfig.to_dict() does not emit a "mnar_constant_fill" key.
• NumericImputationConfig.from_dict() silently ignores a "mnar_constant_fill" key if present in legacy payloads (or simply drops it).
• All docstrings on ImputationStrategy, NumericImputationConfig, and their public methods are numpy-style and up to date with the new field set (per ADR-0034).

Blocked by

None — can start immediately.

[DEVunderdog]

Resolved. Changes made:

• ImputationStrategy.MNAR = "mnar" added to the enum; Constant retained as a dead value for the from_dict() migration shim (feat(imputation): add 'constant' → 'mnar' migration shim in FittedImputer.from_dict() #166).
• mnar_constant_fill field removed from NumericImputationConfig — field, docstring, to_dict(), and from_dict() all updated. from_dict() silently ignores the key if present in legacy payloads.
• ImputationConfig docstring updated: mnar_columns description now reflects data-derived fill instead of Constant fill.
• _numeric_imputer.py Priority 2 block updated to ImputationStrategy.MNAR with a 0.0 placeholder fill value (data-derived computation implemented in feat(imputation): replace sentinel constant fill with data-derived skew-driven fill in _fit_one Priority 2 MNAR block #165).
• All affected tests updated: test_imputation_config.py, test_numeric_imputer.py, test_imputation_end_to_end.py. 326 tests pass.