feat: struct方法 impl (P1 #9)

- lexer: TOK_IMPL 关键字
- ast: AST_IMPL_BLOCK, AST_METHOD_CALL + AST_PROGRAM impls数组
- parser: impl StructName { fn ... } + p.method() 方法调用
- sema: 方法名mangle(StructName$method), self参数, 类型检查
- codegen: METHOD_CALL→mangled函数调用(recv为第一参数)
- 新增集成测试: 19_struct_method.l

P1 4项全部完成: type alias + enum + array + impl
测试: 145 通过 (41+15+65+24)

src/codegen/codegen.c

@@ -368,6 +368,28 @@ static LLVMValueRef codegen_expr(CgCtx* ctx, AstNode* node) {
         return LLVMConstInt(LLVMInt64TypeInContext(ctx->context),
             (unsigned long long)node->as.enum_variant.variant_index, true);
 
+    case AST_METHOD_CALL: {
+        const char* struct_name = node->as.method_call.receiver->type.struct_name;
+        char mangled[256];
+        snprintf(mangled, sizeof(mangled), "%s$%s", struct_name,
+                 node->as.method_call.method_name);
+        LLVMValueRef fn = find_fn(ctx, mangled);
+        if (!fn) return NULL;
+        // 参数列表: [receiver, 用户参数...]
+        LLVMValueRef args[16];
+        args[0] = codegen_expr(ctx, node->as.method_call.receiver);
+        if (!args[0]) return NULL;
+        for (size_t i = 0; i < node->as.method_call.arg_count; i++) {
+            args[i + 1] = codegen_expr(ctx, node->as.method_call.args[i]);
+            if (!args[i + 1]) return NULL;
+        }
+        LLVMTypeRef fn_ty = LLVMGlobalGetValueType(fn);
+        LLVMTypeRef ret_ty = LLVMGetReturnType(fn_ty);
+        return LLVMBuildCall2(ctx->builder, fn_ty, fn, args,
+            (unsigned)(node->as.method_call.arg_count + 1),
+            ret_ty == LLVMVoidTypeInContext(ctx->context) ? "" : "method_call");
+    }
+
     case AST_INDEX_EXPR: {
         // 获取数组变量的指针
         AstNode* arr_node = node->as.index_expr.array;