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/lexer/token.c

@@ -7,7 +7,7 @@
 static const char* NAMES[] = {
     [TOK_FN] = "fn", [TOK_LET] = "let", [TOK_MUT] = "mut", [TOK_IF] = "if",
     [TOK_ELSE] = "else", [TOK_WHILE] = "while", [TOK_FOR] = "for", [TOK_IN] = "in", [TOK_RETURN] = "return",
-    [TOK_STRUCT] = "struct", [TOK_TYPE] = "type", [TOK_ENUM] = "enum",
+    [TOK_STRUCT] = "struct", [TOK_TYPE] = "type", [TOK_ENUM] = "enum", [TOK_IMPL] = "impl",
     [TOK_I64] = "i64", [TOK_F64] = "f64", [TOK_BOOL] = "bool", [TOK_STR] = "str", [TOK_VOID] = "void",
     [TOK_INT_LIT] = "整数", [TOK_FLOAT_LIT] = "浮点数", [TOK_STR_LIT] = "字符串",
     [TOK_TRUE] = "true", [TOK_FALSE] = "false",