Fix ISO 8211 concatenated array descriptor parsing with leader subfields

[philliphoff]

Problem

ParseSubfieldLabels only splits on ! delimiters, so concatenated array label strings like VCID*YCOO!XCOO!ZCOO merged VCID and YCOO into a single name (VCID*YCOO). This produced 3 subfield definitions instead of 4, with RepeatingSubfieldStartIndex = -1.

This breaks S-101 MultiPoint (C3IL) records and any ISO 8211 concatenated array field using leader + repeating subfields separated by * (per ISO 8211 §6.4.3.3).

Fix

Iso8211DataDescriptiveRecordReader.cs — In ParseSubfieldLabels, add handling for mid-string * to split VCID*YCOO into two names: VCID (leader, index 0) and YCOO (repeating start, index 1). The existing *-at-start handling for vector-style fields is preserved.

Iso8211FieldReader.cs — Fix a GroupCount double-counting bug when RepeatingSubfieldStartIndex > 0: the first repeating group was counted both at entry (set to 1) and at wrap-around (incremented). Changed initial value to 0 so wrap-around correctly produces the count.

Tests

• DDR Reader: Concatenated array with leader+repeating produces 4 subfields with correct names, formats, and RepeatingSubfieldStartIndex = 1
• DDR Reader: Regression guard for vector-style *YCOO!XCOO still works correctly
• Field Reader: End-to-end read of C3IL-style field with 1 leader byte + 3 repeating groups verifies GetFixedSubfield, GroupCount, and GetSubfieldGroups
• All 421 existing tests pass