feat: Phase 1 core semantics + Array.prototype methods
Phase 1 - Core semantics fixes:
- Add JsErrorType enum (Error/SyntaxError/TypeError/ReferenceError/RangeError)
- Fix == type coercion via Value::abstract_eq (Abstract Equality Comparison)
- Add strict mode detection via 'use strict' directive
- Fix Object.defineProperty to handle writable/enumerable/configurable
- Fix error types: ReferenceError for undefined vars, TypeError for type mismatches
- Update test262 runner to detect error types from stderr output
Array.prototype new methods:
- splice, unshift, sort, reverse, concat
- reduce, reduceRight, some, every
- find, findIndex, fill, flat, lastIndexOf
- Array.from (static method)
- Plus 20 integration tests in tests/cli.rs