Major refactor — recursive descent parser, arithmetic, unary operators, and token system overhaul

New Features

• Arithmetic operations — Full support for math expressions with proper operator precedence: +, -, *, /, %. Parentheses can be used to override precedence. (2 + 3 * 4 == 14)
• String concatenation — The + operator now performs string concatenation when either operand is a string, and numeric addition otherwise. ("hello " + "world")
• Unary minus and logical NOT operators — Support for negation (-5), double negation (--5), and logical NOT (!false, !(1 == 2), !foo).
• Expression result API — Rule::result() returns the actual computed value of an expression (e.g., int(15) for 5 * 3), not just true/false.
• Syntax highlighter — A built-in custom syntax highlighter with customizable styles for token types.
• Duplicate regex modifier validation — Invalid or duplicate regex modifiers now throw an error.

Major Refactors & Improvements

• Recursive descent parser — Replaced the old parser with a proper recursive descent parser, improving correctness and maintainability.
• Token system simplification — Consolidated 27 token classes into a single GenericToken + TokenKind enum, drastically reducing complexity.
• Lexer streaming — Lexer::tokenize() converted from array-based to generator-based streaming for better memory efficiency.
• Eliminated TokenStream God object — Cleaned up the dependency tree by removing the monolithic TokenStream class.
• RuleEngineBuilder — Extracted dependency wiring from the God constructor in RuleEngine into a dedicated builder.
• Interpreter pattern in AstEvaluator — Replaced fragile match expressions with the Interpreter pattern for cleaner node evaluation.
• Consolidated InternalFunction/InternalMethod — Merged into a single InternalCallable DTO.
• Consolidated Token/TokenKind enums — Reduced duplication across token enums.
• Removed dead code — Cleaned up the Expression directory, EvaluableExpression, unused TokenIterator methods, and more.
• PSR-12 coding standard — Migrated from PSR-2 to PSR-12.

Bug Fixes

• Division by zero — Now properly handled in the evaluator instead of causing errors.
• String escape sequences — Fixed unescaping of escape sequences in TokenEncapsedString.
• AST cache not used — Fixed double parsing in Rule.php where the AST cache was never utilized.
• Infinite recursion — Fixed infinite recursion in Rule.php property hook.
• Lexer reentrancy — Fixed mutable state / reentrancy issue in the Lexer.
• FunctionRegistry performance — Eliminated unnecessary new instance creation on every function call.
• TokenCollection data structure — Replaced SplObjectStorage with ArrayObject to fix wrong data structure usage.
• Fragile method prefix fallback — Refactored fragile method prefix fallback loop in ObjectMethodCaller.
• Double evaluation — Eliminated double evaluation in isValid()/getError().
• Duplicated node evaluation — Fixed duplicated node evaluation logic in AstEvaluator.
• Stack-based token inspection — Replaced fragile $afterValue boolean with stack-based token inspection.

Dependency Updates

• Updated PHP requirement to >=8.5
• Updated phpunit/phpunit to ^13.1
• Updated mockery/mockery to ^1.6
• Updated squizlabs/php_codesniffer to ^4.0