[wasphin]

What problem does this PR solve?

Problem Summary:

Numeric conversion of a bounded splitter field passes its start to C conversion routines without terminating it at the field end. A numeric suffix outside the field can affect conversion.

What is changed and the side effects?

Changed:

• Convert a terminated copy of the current field in both splitter classes.
• Document that conversions use only the current field.
• Cover all six numeric conversion routines with a numeric suffix and a buffer without a trailing NUL.

Side effects:

• Performance effects: Copies the current field for numeric conversion.
• Breaking backward compatibility: Numeric separators now delimit numeric fields in the same way as other separators. No API signature changes.

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Check List:

• Changes compile locally.
• Added focused regression coverage.
• Please follow Contributor Covenant Code of Conduct.