[BUG] - Garbled characters rendering under OpenBSD 7.1

**Describe the bug**

Compiling a Rust cursive application on OpenBSD gives a running executable
but the on terminal rendering is garbled

**To Reproduce**

Build any cursive application and run it under OpenBSD.

**Expected behavior**

The characters used for the UI decoration should display properly.

**Screenshots**


**Environment**
* OpenBSD 7.1

* This bug seems independent of the used back-end.

* locale
LANG=
LC_ALL=
LC_* = "C"

* export TERM=linux

* cursive version used is "0.16.0" for compatibility with OpenBSD 7.1 rust toolchain.

Start by reproducing a Rust cursive application on OpenBSD 7.1 with the reported C locale and TERM=linux settings, testing the available backends because the report says the problem is backend-independent. Trace how terminal UI decoration characters are selected and rendered; done means those characters display correctly without breaking the reported environment.

由索引模型根据 Issue 内容生成。