Full Unicode case-mapping parity for case-insensitive field matching

[goel-skd]

Context

Issue #613 asks for case-insensitive field matching consistent with iceberg-java and iceberg-python (both Unicode-aware), with İ (U+0130) as the example. PR #760 (Part of #613) made StringUtils::ToLower Unicode-aware using utf8proc simple (1:1) case mapping and added allocation-free ASCII fast paths.

This issue captures the design and remaining plan to reach full parity and tracks the follow-up PRs.

Remaining gap

utf8proc's simple mapping still diverges from java for the few code points where simple ≠ full case mapping — chiefly İ:

input	iceberg-cpp (simple)	iceberg-java toLowerCase(Locale.ROOT) / Python str.lower()
İ (U+0130)	i (U+0069)	i̇ (U+0069 U+0307)

So EqualsIgnoreCase("İD", "id") is true in iceberg-cpp but false in java/python — the inconsistency #613 is about.

Design questions

• Match iceberg-java toLowerCase(Locale.ROOT) exactly; confirm the operation PyIceberg uses for matching and that it agrees.
• Full lowercase mapping vs. Unicode case folding: utf8proc offers full case folding (utf8proc_map + UTF8PROC_CASEFOLD); verify it reproduces the java/python result, or add a small explicit mapping.
• Keep the ASCII fast path; stream the non-ASCII path rather than materialize.

Work Items

• Full case mapping to close the İ / java-parity gap
• Streaming / allocation-free non-ASCII comparison in EqualsIgnoreCase / StartsWithIgnoreCase (deferred from feat(string_util): make ToLower Unicode-aware via utf8proc (2/2) #760)

References

• Issue: case insensitive field matching behavior different from iceberg-python and iceberg-java #613 (origin)