feat: splitting/_profile_signals.py string sentinel replace semantics

[DEVunderdog]

Parent

#176

What to build

Update splitting/_profile_signals.py to use user-declared string sentinels with replace semantics when computing effective null masks for split-balance signals.

Where the module currently references _SENTINEL_STRINGS unconditionally for String/Utf8 columns (line 58), it should instead resolve the per-column sentinel set from the profile result:

• If profile_result.string_sentinels has an entry for the column → use only the declared values (case-insensitive)
• If no entry exists → fall back to _SENTINEL_STRINGS (unchanged current behaviour)

Empty/whitespace detection is always applied unconditionally.

StructuralProfileResult.string_sentinels (added in #177) is the source of the declarations — the split module already receives the profile result.

Acceptance criteria

• Effective null mask for a String/Utf8 column with a declared sentinel uses only the declared values when computing split signals
• Effective null 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
• SplitImbalanceWarning fires correctly when sentinel-based missingness lands disproportionately in one split, using declared sentinels
• Columns of other dtypes are completely unaffected
• Unit tests covering the replace-semantics path in the relevant split-signal test file
• Docstrings updated per ADR-0034 for any modified in-scope functions

Blocked by

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