fix(protogen): add correct cell positioners for struct and union type sheet parsing

[Kybxd]

• close protogen: panic on wrapDebugErr of v0.15.1 #371

Problem

When parsing struct type and union type sheets (e.g., #StructDefault, #UnionDefault), the error messages reported incorrect cell positions (NameCellPos / TypeCellPos). For example:

• In a struct type sheet with vertical layout, a field at row 6 was incorrectly reported as D1 / D2 instead of A6 / B6.
• In a union type sheet, a field at row 228 column D was incorrectly reported as A228 instead of D228.

This made it difficult for users to locate the actual problematic cell in the Excel file.

Root Cause

• Struct type sheets lay out fields vertically (one field per row), but the default positioner treated the cursor as a column index.
• Union type sheets embed field name/type/note in multi-line cells within each value row, but had no positioner to correctly map cursor to the actual Field column.

Solution

• Added verticalPositioner for struct type sheets: maps (headerRow, cursor) → (dataRow + cursor, headerRow), correctly translating field index to data row position.
• Added unionFieldPositioner for union type sheets: maps cursor to the actual column starting from Field1, at the specific union value row.
• Added findFieldStartCol helper to locate the Field1 column index in the header row, supporting varying header layouts (with/without Number, Type columns).
• Both positioners work correctly with transposed tables.

Tests

Added comprehensive unit tests in sheet_mode_test.go covering:

• verticalPositioner with normal and transposed tables
• unionFieldPositioner with normal and transposed tables
• findFieldStartCol with various header layouts including transposed tables

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.98%. Comparing base (bc70718) to head (8300437).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #372      +/-   ##
==========================================
+ Coverage   73.91%   73.98%   +0.06%     
==========================================
  Files          87       87              
  Lines        8664     8686      +22     
==========================================
+ Hits         6404     6426      +22     
  Misses       1700     1700              
  Partials      560      560              

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