feat: MissingnessProfiler string sentinel replace semantics

[DEVunderdog]

Parent

#176

What to build

Update MissingnessProfiler._profile_column in profiling/_missingness_profiler.py to accept an optional string_sentinels: list[str] | None parameter (default None).

When string_sentinels is provided and the column is String/Utf8, the eff_null boolean mask uses only the declared values (case-insensitive) for sentinel matching — the hardcoded _SENTINEL_STRINGS set is not applied for that column. Empty/whitespace-only strings are always included in the mask regardless of any declaration.

_run() passes profile_config.string_sentinels.get(col_name) for each column so the profiler reads declared sentinels from config without structural changes to its call signature from the orchestrator.

When string_sentinels=None, output is identical to the current behaviour — hardcoded _SENTINEL_STRINGS apply.

Acceptance criteria

• _profile_column gains string_sentinels: list[str] | None = None
• Declared sentinel values are counted in effective_null_count alongside standard Polars nulls
• Hardcoded defaults are not counted for a column that has a declaration (replace semantics)
• Empty/whitespace strings are counted regardless of whether a declaration exists
• effective_null_ratio reflects the declaration-inclusive count
• Severity classification (Minor/Moderate/High/Severe) uses the declaration-inclusive ratio
• A column with >50% declared-sentinel rows receives a DropCandidate flag
• Declared sentinel rows participate in the missingness indicator used for MAR correlation detection
• When string_sentinels=None, output is identical to current behaviour (no regression)
• Columns with no declaration are completely unaffected
• Unit tests in tests/unit/profiling/test_missingness_profiler.py covering all criteria above
• Docstrings updated per ADR-0034

Blocked by

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