feat: 结构体 struct — 最后一项 P0 功能

- lexer: TOK_STRUCT, TOK_DOT 关键字和运算符
- ast: AST_STRUCT_DECL/STRUCT_INIT/FIELD_ACCESS 3 种新节点
- parser: struct 声明 + .field 访问 + Name{field:val} 初始化
- sema: struct 类型符号表，字段类型解析，初始化字段检查
- codegen: LLVMStructType + extractvalue/insertvalue 字段操作
- 新增集成测试: 12_struct.l, 13_struct_nested.l
- 基于 Codex 分析报告 P0 #4

所有 P0 功能已全部完成。

src/ast/ast.h

@@ -20,6 +20,9 @@ typedef enum {
     AST_CALL_EXPR,
     AST_LITERAL_EXPR,
     AST_IDENT_EXPR,
+    AST_STRUCT_DECL,      // struct Point { x: i64, y: i64 }
+    AST_STRUCT_INIT,      // Point { x: 10, y: 20 }
+    AST_FIELD_ACCESS,     // p.x
 } AstKind;
 
 typedef enum {
@@ -32,6 +35,7 @@ typedef enum {
 // 类型信息（语义分析阶段填充）
 typedef struct {
     TypeKind kind;
+    const char* struct_name;  // TYPE_STRUCT 时的结构体类型名
 } TypeInfo;
 
 // AST 节点
@@ -44,16 +48,18 @@ struct AstNode {
     // 节点特有数据（按 kind 解释）
     union {
         // AST_PROGRAM
-        struct { struct AstNode** functions; size_t fn_count; } program;
+        struct { struct AstNode** functions; size_t fn_count;
+                 struct AstNode** structs;   size_t struct_count; } program;
         // AST_FUNCTION
         struct { const char* name; struct AstNode** params; size_t param_count;
                  TypeKind return_type; struct AstNode* body; } function;
-        // AST_PARAMETER
-        struct { const char* name; TypeKind type; } parameter;
+        // AST_PARAMETER (也用作结构体字段: name + type)
+        struct { const char* name; TypeKind type; const char* struct_type_name; } parameter;
         // 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; } let_stmt;
+        struct { const char* name; TypeKind annot_type; bool has_type_annot; bool is_mut; struct AstNode* init;
+                 const char* struct_type_name; } let_stmt;
         // AST_ASSIGN_STMT
         struct { const char* name; struct AstNode* value; } assign_stmt;
         // AST_IF_STMT
@@ -74,16 +80,25 @@ struct AstNode {
         struct { TypeKind lit_type; union { int64_t i64_val; double f64_val; bool bool_val; const char* str_val; }; } literal;
         // AST_IDENT_EXPR
         struct { const char* name; } ident;
+        // AST_STRUCT_DECL
+        struct { const char* name; struct AstNode** fields; size_t field_count; } struct_decl;
+        // AST_STRUCT_INIT
+        struct { const char* type_name; const char** field_names;
+                 struct AstNode** field_values; size_t field_count; } struct_init;
+        // AST_FIELD_ACCESS
+        struct { struct AstNode* object; const char* field; int field_index; } field_access;
     } as;
 };
 
 // 创建节点的辅助函数（内存来自 arena，通过 void* 传递避免循环依赖）
-AstNode* ast_make_program(void* alloc, AstNode** fns, size_t count, int line, int col);
+AstNode* ast_make_program(void* alloc, AstNode** fns, size_t fn_count,
+                          AstNode** structs, size_t struct_count, int line, int col);
 AstNode* ast_make_function(void* alloc, const char* name, AstNode** params, size_t pcount,
                            TypeKind ret, AstNode* body, int line, int col);
-AstNode* ast_make_parameter(void* alloc, const char* name, TypeKind type, int line, int col);
+AstNode* ast_make_parameter(void* alloc, const char* name, TypeKind type, const char* struct_type_name, int line, int col);
 AstNode* ast_make_block(void* alloc, AstNode** stmts, size_t count, int line, int col);
-AstNode* ast_make_let(void* alloc, const char* name, TypeKind annot_type, bool has_type_annot, bool is_mut, AstNode* init, int line, int col);
+AstNode* ast_make_let(void* alloc, const char* name, TypeKind annot_type, bool has_type_annot,
+                      bool is_mut, AstNode* init, const char* struct_type_name, int line, int col);
 AstNode* ast_make_assign(void* alloc, const char* name, AstNode* value, int line, int col);
 AstNode* ast_make_if(void* alloc, AstNode* cond, AstNode* then_b, AstNode* else_b, int line, int col);
 AstNode* ast_make_while(void* alloc, AstNode* cond, AstNode* body, int line, int col);
@@ -97,5 +112,8 @@ AstNode* ast_make_literal_f64(void* alloc, double val, int line, int col);
 AstNode* ast_make_literal_bool(void* alloc, bool val, int line, int col);
 AstNode* ast_make_literal_str(void* alloc, const char* val, int line, int col);
 AstNode* ast_make_ident(void* alloc, const char* name, int line, int col);
+AstNode* ast_make_struct_decl(void* alloc, const char* name, AstNode** fields, size_t count, int line, int col);
+AstNode* ast_make_struct_init(void* alloc, const char* type_name, const char** fnames, AstNode** fvals, size_t count, int line, int col);
+AstNode* ast_make_field_access(void* alloc, AstNode* object, const char* field, int line, int col);
 
 #endif