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)

test/test_sema.c

@@ -329,6 +329,42 @@ void test_array_assign_ok() {
     arena_destroy(&a);
 }
 
+/* === 方法调用语义分析测试 === */
+
+void test_method_call_ok() {
+    Arena a = arena_create(1);
+    size_t tc; ErrorInfo lex_err = {0};
+    Token* toks = lex(&a,
+        "struct Point { x: i64, y: i64 } impl Point { fn get_x(self: Point) -> i64 { return self.x; } } fn main() -> i64 { let p: Point = Point { x: 42, y: 0 }; return p.get_x(); }",
+        "test", &tc, &lex_err);
+    ASSERT(toks != NULL);
+    ErrorInfo parse_err = {0};
+    AstNode* ast = parse(&a, toks, tc, "test", &parse_err);
+    ASSERT(ast != NULL);
+
+    ErrorList errors; error_init(&errors);
+    sema_analyze(ast, &errors, &a);
+    ASSERT(errors.count == 0);
+    arena_destroy(&a);
+}
+
+void test_method_undefined() {
+    Arena a = arena_create(1);
+    size_t tc; ErrorInfo lex_err = {0};
+    Token* toks = lex(&a,
+        "struct Point { x: i64, y: i64 } fn main() -> i64 { let p: Point = Point { x: 42, y: 0 }; return p.nope(); }",
+        "test", &tc, &lex_err);
+    ASSERT(toks != NULL);
+    ErrorInfo parse_err = {0};
+    AstNode* ast = parse(&a, toks, tc, "test", &parse_err);
+    ASSERT(ast != NULL);
+
+    ErrorList errors; error_init(&errors);
+    sema_analyze(ast, &errors, &a);
+    ASSERT(errors.count > 0);  // nope 不是 Point 的方法
+    arena_destroy(&a);
+}
+
 int main(void) {
     TEST_RUN(test_type_error);
     TEST_RUN(test_undefined_var);
@@ -349,5 +385,7 @@ int main(void) {
     TEST_RUN(test_array_index_type_error);
     TEST_RUN(test_array_not_indexable);
     TEST_RUN(test_array_assign_ok);
+    TEST_RUN(test_method_call_ok);
+    TEST_RUN(test_method_undefined);
     return test_summary();
 }