fix: keep sheet multi-header record fields

[fangsmile]

Summary

• fix VTableSheet column normalization so mixed leaf/grouped headers preserve user-defined record fields
• add an issue 5184 reproduction demo for multi-level headers with record data
• add a regression test covering mixed top-level leaf and grouped columns

Fixes #5184

Tests

• rushx test --runInBand multi-header-record-data.test.ts
• rushx compile
• rushx test --runInBand
• git diff --check