feat: profiling/orchestrator.py MAR detection — string sentinel replace semantics

[DEVunderdog]

Parent

#176

What to build

Update the MAR correlation detection loop in profiling/orchestrator.py to use user-declared string sentinels with replace semantics when building per-column missingness masks.

Where the orchestrator currently references _SENTINEL_STRINGS unconditionally for String/Utf8 columns (around lines 357 and 375), it should instead check profile_config.string_sentinels.get(col_name):

• If a declaration exists for the column → use only the declared values (case-insensitive) for sentinel matching in the mask
• If no declaration exists → fall back to _SENTINEL_STRINGS (unchanged current behaviour)

Empty/whitespace detection is always applied unconditionally.

Acceptance criteria

• MAR correlation missingness mask for a String/Utf8 column with a declared sentinel uses only the declared values, not _SENTINEL_STRINGS
• MAR correlation missingness mask for a String/Utf8 column with no declaration continues to use _SENTINEL_STRINGS (no regression)
• Empty/whitespace strings are always included in the mask regardless of declaration
• Columns of other dtypes are completely unaffected
• Unit tests in tests/unit/profiling/test_pipeline_config.py or test_missingness_profiler.py (whichever already covers MAR detection paths) verify the replace-semantics mask
• Docstrings updated per ADR-0034 for any modified in-scope methods

Blocked by

• Blocked by feat: ProfileConfig.string_sentinels declaration + StructuralProfileResult propagation #177