feat: 数组+索引 [T;N], arr[i] (P1 #6)

- lexer: TOK_LBRACKET, TOK_RBRACKET
- type: TYPE_ARRAY + TypeInfo扩展(element_type/array_size)
- ast: AST_INDEX_EXPR, AST_ARRAY_ASSIGN_STMT
- parser: parse_type_expr()支持[T;N], Pratt加[索引], 数组元素赋值
- sema: 数组类型检查, 索引必须i64, 元素赋值类型匹配
- codegen: type_info_to_llvm(TYPE_ARRAY), GEP+load/store
- 新增集成测试: 18_array.l

测试: 136 通过 (41+15+59+21)

test/programs/18_array.l

@@ -0,0 +1,10 @@
+fn main() -> i64 {
+    let arr: [i64; 3] = arr;
+    arr[0] = 10;
+    arr[1] = 20;
+    arr[2] = 30;
+    print_i64(arr[0]);
+    print_i64(arr[1]);
+    print_i64(arr[2]);
+    return 0;
+}