Enhancement: Use string_view to reduce string allocations

[clank-hayen]

Problem

The lexer and parser create many temporary strings during parsing. For example:

// In lexer.cpp
while (isalnum(current()) || current() == '_') {
    value += current();  // String allocation per character
    advance();
}

// In parser - string concatenation
string qualified_name = ident + "." + member_name;  // Allocation
decl->type = "List<" + element_type + ">";          // Allocation

Suggested Solution

Use `std::string_view` where possible:

1. Lexer: Return views into the source string

   struct Token {
       TokenType type;
       string_view value;  // View into source
       int line;
   };

2. Parser lookups: Use string_view for comparisons

   bool is_primitive_type(string_view type);
   const StructDef* get_struct(string_view name);

3. Type extraction: Return views for type components

   string_view extract_element_type(string_view generic_type, string_view prefix);

Trade-offs

Pros:

• Reduced allocations during parsing
• Faster keyword/type comparisons
• Lower memory pressure

Cons:

• Must ensure source string lifetime exceeds tokens
• Some APIs may still need owned strings

Priority

Low - This is a performance optimization. Profile first to confirm benefit.