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.c

@@ -4,13 +4,16 @@
 // 使用宏简化节点创建
 #define NEW(alloc, k) \
     AstNode* n = (AstNode*)arena_alloc_impl(alloc, sizeof(AstNode)); \
-    n->kind = (k); n->type.kind = TYPE_UNKNOWN; \
+    n->kind = (k); n->type.kind = TYPE_UNKNOWN; n->type.struct_name = NULL; \
     n->line = line; n->col = col
 
-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) {
     NEW(alloc, AST_PROGRAM);
     n->as.program.functions = fns;
-    n->as.program.fn_count = count;
+    n->as.program.fn_count = fn_count;
+    n->as.program.structs = structs;
+    n->as.program.struct_count = struct_count;
     return n;
 }
 
@@ -23,9 +26,11 @@ AstNode* ast_make_function(void* alloc, const char* name, AstNode** params, size
     return n;
 }
 
-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) {
     NEW(alloc, AST_PARAMETER);
     n->as.parameter.name = name; n->as.parameter.type = type;
+    n->as.parameter.struct_type_name = struct_type_name;
     return n;
 }
 
@@ -35,10 +40,13 @@ AstNode* ast_make_block(void* alloc, AstNode** stmts, size_t count, int line, in
     return n;
 }
 
-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) {
     NEW(alloc, AST_LET_STMT);
     n->as.let_stmt.name = name; n->as.let_stmt.annot_type = annot_type;
-    n->as.let_stmt.has_type_annot = has_type_annot; n->as.let_stmt.is_mut = is_mut; n->as.let_stmt.init = init;
+    n->as.let_stmt.has_type_annot = has_type_annot; n->as.let_stmt.is_mut = is_mut;
+    n->as.let_stmt.init = init;
+    n->as.let_stmt.struct_type_name = struct_type_name;
     return n;
 }
 
@@ -124,3 +132,34 @@ AstNode* ast_make_ident(void* alloc, const char* name, int line, int col) {
     n->as.ident.name = name;
     return n;
 }
+
+// === 结构体相关工厂函数 ===
+
+AstNode* ast_make_struct_decl(void* alloc, const char* name, AstNode** fields,
+                               size_t count, int line, int col) {
+    NEW(alloc, AST_STRUCT_DECL);
+    n->as.struct_decl.name = name;
+    n->as.struct_decl.fields = fields;
+    n->as.struct_decl.field_count = count;
+    return n;
+}
+
+AstNode* ast_make_struct_init(void* alloc, const char* type_name,
+                               const char** fnames, AstNode** fvals,
+                               size_t count, int line, int col) {
+    NEW(alloc, AST_STRUCT_INIT);
+    n->as.struct_init.type_name = type_name;
+    n->as.struct_init.field_names = fnames;
+    n->as.struct_init.field_values = fvals;
+    n->as.struct_init.field_count = count;
+    return n;
+}
+
+AstNode* ast_make_field_access(void* alloc, AstNode* object, const char* field,
+                                int line, int col) {
+    NEW(alloc, AST_FIELD_ACCESS);
+    n->as.field_access.object = object;
+    n->as.field_access.field = field;
+    n->as.field_access.field_index = -1;
+    return n;
+}