feat: const 编译期常量 — const N = 42;

Token(77): +TOK_CONST, AST_LET_STMT 新增 is_const, Symbol 新增 const_value
sema: 字面量初始化自动折叠为编译期常量值

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ast/ast.h

@@ -78,7 +78,7 @@ struct AstNode {
         // AST_BLOCK
         struct { struct AstNode** stmts; size_t stmt_count; } block;
         // AST_LET_STMT
-        struct { const char* name; TypeKind annot_type; bool has_type_annot; bool is_mut; struct AstNode* init;
+        struct { const char* name; TypeKind annot_type; bool has_type_annot; bool is_mut; bool is_const; struct AstNode* init;
                  const char* struct_type_name;
                  TypeKind annot_element_type; const char* annot_element_struct_name; int64_t annot_array_size; } let_stmt;
         // AST_ASSIGN_STMT