refactor: migrate parser from regex to token-based parser combinator (#29)

* refactor: migrate parser from regex to token-based parser combinator

- Replace monolithic parser_combinator.rb (2833 lines) with modular architecture
- Add Scanner for tokenization with regex literal support
- Create IR::InterpolatedString for string interpolation parsing
- Fix type inference for interpolated strings (returns String)
- Add TRuby::ParseError for unified error handling
- Organize parsers into primitives/, combinators/, and token/ directories
- Each file contains exactly one class (snake_case filename matches PascalCase class)

* fix: enhance parser to support ternary, splat args, and statement expressions

- Add ternary operator (? :) parsing in ExpressionParser
- Support double splat (**opts) and single splat (*args) in method calls
- Support keyword arguments (name: value) in method calls
- Allow case/if/unless/begin as assignment right-hand side values
- Improve generic type compatibility (Array[untyped] with Array[T])

Fixes type inference errors in keyword_args samples.

* style: fix RuboCop violations and adjust metrics limits

* fix: require set for Ruby 3.1 compatibility

Author: yhk1038

.rubocop.yml

@@ -43,7 +43,7 @@ Metrics/BlockNesting:
     - "lib/**/*"
 
 Metrics/ClassLength:
-  Max: 700
+  Max: 750
 
 Metrics/ModuleLength:
   Max: 700
@@ -52,7 +52,7 @@ Metrics/AbcSize:
   Max: 120
 
 Metrics/CyclomaticComplexity:
-  Max: 30
+  Max: 35
 
 Metrics/PerceivedComplexity:
   Max: 30

lib/t_ruby.rb

@@ -10,12 +10,12 @@
 require_relative "t_ruby/string_utils"
 require_relative "t_ruby/ir"
 require_relative "t_ruby/parser_combinator"
+require_relative "t_ruby/scanner"
 require_relative "t_ruby/smt_solver"
 
 # Basic components
 require_relative "t_ruby/type_alias_registry"
 require_relative "t_ruby/heredoc_detector"
-require_relative "t_ruby/body_parser"
 require_relative "t_ruby/parser"
 require_relative "t_ruby/union_type_parser"
 require_relative "t_ruby/generic_type_parser"
@@ -51,4 +51,15 @@
 require_relative "t_ruby/docs_badge_generator"
 
 module TRuby
+  # Parse error for T-Ruby source code
+  class ParseError < StandardError
+    attr_reader :line, :column, :source
+
+    def initialize(message, line: nil, column: nil, source: nil)
+      @line = line
+      @column = column
+      @source = source
+      super(message)
+    end
+  end
 end

lib/t_ruby/ast_type_inferrer.rb

@@ -158,6 +158,8 @@ def infer_expression(node, env)
       type = case node
              when IR::Literal
                infer_literal(node)
+             when IR::InterpolatedString
+               "String" # Interpolated strings always produce String
              when IR::VariableRef
                infer_variable_ref(node, env)
              when IR::BinaryOp