Generate complete static case fold orbits

[ittaigolde]

Fixes #168.

This builds on #170's Unicode 10 table update and makes the fix independent of the runtime JDK's Unicode version.

I reproduced the hang on JDK 21 and scanned all Unicode codepoints with a bounded simpleFold() orbit walker. On master, U+1C80..U+1C88 produce non-closing orbits. For example:

U+1C80 -> U+0412 -> U+0432 -> U+0412 ...

#170 fixes the currently observed failures, but simpleFold() still mixes generated ICU tables with the active JDK's Character.toLowerCase() / toUpperCase() data. A later JDK could therefore introduce another incomplete orbit.

Following @herbyderby's suggestion in #168, this change:

• generates complete static fold orbits, including ordinary upper/lower pairs;
• stores them in sparse 256-entry int[] pages, supporting supplementary codepoints;
• removes the runtime JDK casing fallback from simpleFold();
• adds bounded closure coverage and public API regressions for the Cyrillic Extended-C case, folded character classes, and a supplementary-plane mapping.

Preliminary JDK 21 microbenchmark, 20 million mixed lookups:

Current hybrid fallback:    7.31 ns/op
Dense complete int[]:       4.80 ns/op
Sorted-array binary search: 14.30 ns/op
Paged sparse int[][]:       4.66 ns/op

Estimated orbit-table memory:

Current #170 char[]:     85 KB
Dense complete int[]:   501 KB
Sorted parallel arrays:  23 KB
Paged sparse int[][]:    29 KB

These are informal measurements rather than JMH results.

Verification:

• compiled main sources directly with JDK 21;
• passed UnicodeTest, PatternTest, and CharClassTest;
• passed all JUnit classes except ExecTest and GWTTest through a standalone Gradle 9 runner.

The excluded classes could not be validated faithfully in that runner on Windows: ExecTest fixture parsing is affected by checkout line endings, and GWTTest expects the legacy root build's resource layout. The repository's Gradle 5.2 wrapper does not start on JDK 21.

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