[I] Add example Variant objects where field_id_size and field_offset_size differ [parquet-testing]

["CurtHagenlocher (via GitHub)" <gi...@apache.org>]

CurtHagenlocher opened a new issue, #124:
URL: https://github.com/apache/parquet-testing/issues/124

   The object `value_header` holds `field_offset_size_minus_one` in bits 0-1 and `field_id_size_minus_one` in bits 2-3. Writers compute the two independently -- field ID width from the largest field ID in the object, offset width from the size of the field data -- so the two routinely differ.
   
   Every object example in `variant/` uses 1-byte field IDs and 1-byte field offsets. When the two are equal, the two header fields hold the same value, so a reader that reads them from each other's bit positions still decodes correctly. The corpus therefore cannot detect that class of bug. arrow-dotnet shipped exactly this bug and validated cleanly against this corpus the whole time (apache/arrow-dotnet#420).
   
   Two examples would close the gap:
   
   - Few fields (1-byte field IDs) but more than 256 bytes of field data (2-byte field offsets) -- header byte `0x06`.
   - A metadata dictionary of more than 256 entries (2-byte field IDs) containing a small object whose own field data is under 256 bytes (1-byte field offsets) -- header byte `0x12`.
   
   Related to #78 but not covered by it: #78 is framed around field *count* and treats the two widths as one knob ("a 3 byte field id / offset"). The first
   example above has only two fields. The second overlaps with #78's ">2^8 fields" case.


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