feat: ImputationOrchestrator.fit() string sentinel threading + end-to-end integration test

[DEVunderdog]

Parent

#176

What to build

Wire string sentinel declarations from the profile into Phase 2 and verify the full pipeline with an integration test.

ImputationOrchestrator.fit() (imputation/orchestrator.py):

• Change the existing _resolve_effective_nulls(train_df) call to _resolve_effective_nulls(train_df, string_sentinels=profile.string_sentinels) so replace-semantics normalization applies to the training split before any fill values are computed.
• Pass string_sentinels=profile.string_sentinels to the FittedImputer constructor so transform-time normalization uses the same declarations.

Integration test (tests/integration/test_imputation_end_to_end.py):

• DataFrame with a String/Utf8 column (status) containing a mix of real values, declared sentinels ("N/A", "missing"), and the hardcoded default "?".
• ProfileConfig declares string_sentinels={"status": ["N/A", "missing"]}.
• StructuralProfiler.profile() runs; the returned profile carries the declarations.
• ImputationOrchestrator.fit() produces a FittedImputer.
• FittedImputer.transform() on the test split produces a result where:
  • "N/A" and "missing" values are filled (treated as effective null).
  • "?" values are not filled (replace semantics: "?" is not in the declaration and the hardcoded set does not apply for this column).
  • The fill value is derived exclusively from non-sentinel observations.

Acceptance criteria

• ImputationOrchestrator.fit() passes string_sentinels=profile.string_sentinels to _resolve_effective_nulls
• ImputationOrchestrator.fit() passes string_sentinels=profile.string_sentinels to the FittedImputer constructor
• Unit test verifies that the FittedImputer returned by fit() has string_sentinels equal to profile.string_sentinels
• Integration test: "N/A" and "missing" are filled in the transform output
• Integration test: "?" is not filled (replace semantics confirmed)
• Integration test: fill value is computed from non-sentinel observations only
• Columns with no declared sentinels are completely unaffected — existing integration tests continue to pass unchanged
• Docstrings updated per ADR-0034

Blocked by

• Blocked by feat: ProfileConfig.string_sentinels declaration + StructuralProfileResult propagation #177
• Blocked by feat: extend _resolve_effective_nulls with user-declared string sentinel replace semantics #178
• Blocked by feat: FittedImputer string_sentinels field, transform wiring, and serialization round-trip #182