Variant field names not being ordered correctly via UTF-8 byte order

### Describe the bug, including details regarding any error messages, version, and platform.

The Variant spec requires the field ids in an object's header to be sorted by the
**UTF-8 byte order** of the field names, so a reader can binary-search them.
`VariantBuilder` sorted the fields — and `Variant.getFieldByKey` binary-searched them —
using `String.compareTo`, which orders by **UTF-16 code units**, not UTF-8 bytes.

The two orderings are identical for all field names in the Basic Multilingual Plane, but
they diverge for names containing supplementary-plane characters (U+10000 and above):
`String.compareTo` orders a leading high surrogate (0xD800–0xDBFF) before code points in
U+E000..U+FFFF, whereas UTF-8 byte order (and the spec) orders them after. Consequences:

- An object parquet-java builds with such keys has field ids sorted in an order that
violates the spec.
- A spec-compliant reader (e.g. the Apache Arrow C++/Rust/Go Variant readers) binary-searching
that object can fail to find fields.
- Conversely, parquet-java's own binary search fails to find a supplementary-plane key in an
object produced by a spec-compliant writer.

The bug only surfaces when an object both contains a supplementary-plane key and is large
enough to take the binary-search path, so it has gone unnoticed.

### Component(s)
Core

VariantBuilder と Variant.getFieldByKey から始め、補助平面のフィールド名を持ち、二分探索を発生させるのに十分な数のフィールドを持つオブジェクトを使って問題を再現します。フィールド ID が UTF-8 のバイト順に従っていること、および parquet-java と仕様準拠の writers の両方で生成されたオブジェクトで検索が成功することを確認します。

索引モデルが issue の本文から書いたものです。