Address code review feedback: fix critical bugs and modernize C++ idioms

[Copilot]

Addresses 44 review comments on PR #20, focusing on P0/P1 bugs and C++ modernization.

Critical Fixes (P0)

ContextParser compilation failures:

• Added missing <memory> include for std::unique_ptr
• Fixed IState.hpp include path (states/base/IState.hpp)
• Fixed TopNodeAs() to cast NodeEntry::MutableNode() instead of NodeEntry&

// Before: doesn't compile
return dynamic_cast<T*>(node_stack_.back());

// After: properly casts the AstNode pointer
return dynamic_cast<T*>(node_stack_.back().MutableNode());

Missing Accept() implementations:

• Implemented visitor dispatch for all 40+ AST node types

Type Safety & Modern C++

Primitive types aligned with VM:

• CharLit: char32_t → char (UTF-8 encoding)
• FloatLit: long double → double (64-bit consistency)
• IntLit: long long → int64_t (explicit width)
• StringLit: std::u32string → std::string

Eliminated raw pointers:

• Operator resolvers: const T* → std::optional<std::reference_wrapper<const T>>
• Diagnostic severity: raw pointer → std::shared_ptr<const ISeverity>
• DiagnosticCollector limits: 0 = unlimited → std::optional<std::size_t>

API improvements:

• Operator tag Name(): const char* → std::string_view
• OpTags: static class → namespace
• ReturnStmt::value_: nullable pointer → std::optional<std::unique_ptr<Expr>>

Structural Improvements

• Added AstNodeDerived concept for TopNodeAs<T>() template constraint
• Enhanced Diagnostic with proper methods, copy/move semantics, and [[nodiscard]]
• Restructured IfStmt/Branch to support else-if chains cleanly
• Specialized InterfaceDecl::members_ to std::vector<std::unique_ptr<InterfaceMethod>>
• Added namespace closures to all parser files with updated header guards

Not Addressed

Deferred design questions requiring broader discussion:

• Renaming Stmt → Statement (230+ files)
• Specializing Call::callee_ to hypothetical CallableExpr base
• Making Unary::op_ a const reference (constructor impact)

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