refactor(regex): make Joni property folding explicit

[fglock]

Summary

• extend forked Joni's character-property resolver result with explicit case-fold policy and character-class context
• retain Perl General_Category fold closure while preserving standalone Block, Script, and Script_Extensions membership under /i
• pass standalone pinned properties to Joni unchanged instead of expanding them in the adapter
• keep no-fold properties inside composed character classes adapter-owned until Joni can preserve per-member policy through class algebra
• document the current native/adapter boundary in the feature matrix

Stack

This draft is stacked on PR #1052 (fix/phase36-branch-reset-named-call) and must not merge first. It is independent of PR #1042's progressive-backend release repair.

Validation

• standard Perl 5.42.2 oracle: 8/8
• warning-free full make: 17 tasks, 5m09s, including direct Joni tests, all unit shards, packaging, and shadow JAR
• forced-Joni focused property matrix on JVM: 194 assertions, 193 pass plus one pre-existing TODO, exit 0
• forced-Joni focused property matrix on interpreter: 194 assertions, 193 pass plus one pre-existing TODO, exit 0
• existing unicode_script_extensions_pinned.t, unicode_block_data.t, unicode_property_aliases.t, and invalid-property diagnostics included in both focused runs
• git diff --check

The remaining character-class work is intentionally not approximated: Joni currently folds a completed class as a single set, so union/intersection/negation need an AST representation that retains each member's fold policy before no-fold properties can move out of the adapter there.