Add example Variant objects where field_id_size and field_offset_size differ

[CurtHagenlocher]

Closes #124.

What

Adds two object examples in variant/:

case	shape	header byte	widths
object_wide_offsets	2 fields, >256 bytes of field data	0x06	1-byte field IDs, 2-byte field offsets
object_wide_field_ids	262 distinct field names; small object under nested	0x12 (nested object)	2-byte field IDs, 1-byte field offsets

These are the first examples in which an object's field_id_size and field_offset_size differ. Every existing object example uses 1-byte field IDs and 1-byte field offsets, where the two value_header fields hold the same value, so a reader that reads them from each other's bit positions still decodes correctly. arrow-dotnet shipped exactly that bug and validated cleanly against this corpus the whole time (apache/arrow-dotnet#420).

object_wide_field_ids has 261 fields, which also makes it the first example with is_large set to 1 (a 4-byte num_elements).

Verification

• regen.py was run end to end and reproduces the four new files byte-identically, along with 49 of the 52 files it generates. See below for the three that differ.
• Both new examples decode to the expected JSON under an independent spec-literal decoder, which was first cross-checked against the existing object_nested and array_nested examples.
• Both are rejected by a reader that transposes the two value_header fields: object_wide_offsets fails its end-offset check, object_wide_field_ids runs off the end of the buffer.

Notes for reviewers

• The JSON for these two cases is built in Python and substituted into the SQL, since 262 field names and a 300-byte string are not readable as inline literals.
• I did not regenerate the rest of the corpus, and data_dictionary.json is edited in place rather than rewritten, since the committed file is hand-maintained (it carries a trailing comma, and primitive_timestampntz already precedes primitive_timestamp_nanos).
• object_wide_field_ids also covers the first case listed in Add example Variant values with larger numbers of fields / array elements #78. The 2**16 and 2**24 cases there remain open; their TODOs in regen.py now sit under a reference to Add example Variant values with larger numbers of fields / array elements #78 rather than Add example nested Variant values that are not JSON encodeable #77, and are reworded, since the number of distinct fields drives the field ID width rather than the field offset width.

Two pre-existing regen.py deviations, unrelated to this change

Noting these since I ran the script; happy to file them separately if useful.

• primitive_null.metadata and primitive_null.value come out empty, which README "Modification 1" already documents as needing a manual fix-up after every regen.
• primitive_timestamp.value is timezone dependent. '2025-04-16T12:34:56.78'::Timestamp is interpreted in the Spark session timezone, so the committed file decodes to 2025-04-16 16:34:56Z (generated under UTC-4) while a run on a US Pacific machine produces 19:34:56Z. Anyone regenerating the corpus will silently rewrite this file.