fix: C backend String functions, record type aliases, docs cleanup
- Add String.fromChar, chars, substring, toUpper, toLower, replace, startsWith, endsWith, join to C backend - Fix record type alias unification by adding expand_type_alias and unify_with_env functions - Update docs to reflect current implementation status - Clean up outdated roadmap items and fix inconsistencies - Add comprehensive language comparison document Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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