Add upstream unitstd Ruby parity lane

[fullofcaffeine]

Summary

• add a curated upstream Haxe unitstd fixture lane for Ruby, starting with StringBuf
• add sync/provenance docs plus the npm run test:unitstd-ruby CI command
• lower String.fromCharCode directly to Ruby and route Haxe substr through HXRuby.string_substr for std parity

Verification

• npm run test:unitstd-ruby
• npm run test:runtime-core
• npm run test:stdlib-mvp
• npm run test:stdlib-inventory
• npm run test:gap-report
• npm run test:snapshots
• npm run format:haxe:check

Note: haxe.ruby-bjv.13 remains open because the broad high-leverage unitstd matrix is manifest-tracked but not fully enabled yet.