RTL text with multi-line <tspan dy> layout collapses onto one line (usvg 0.47.0)

Nobody has claimed this yet.

When a single `` element lays out multiple lines using per-`` dy offsets, and the text is RTL (Arabic/Persian, Hebrew, …), usvg positions the glyphs incorrectly: all lines collapse onto the first baseline and only the logically-first character of each subsequent `` receives the vertical offset. The same structure with LTR text lays out correctly.
This is a layout/positioning bug only — glyph selection and shaping (joining forms, ZWNJ, combining marks) are correct.

Example SVG:
```

```

Example to reproduce:
```
let mut opt = usvg::Options::default();
opt.fontdb_mut().load_system_fonts();

let tree = usvg::Tree::from_str(INPUT_SVG, &opt).unwrap();

// Freeze usvg's layout: glyphs become s at the computed positions.
let flattened = tree.to_string(&usvg::WriteOptions {
preserve_text: false,
..Default::default()
});
std::fs::write("flattened.svg", flattened).unwrap();
```

[A minimal example, but generated by AI.](https://github.com/user-attachments/files/30053791/usvg-example.zip)

One possible workaround is to use `y` instead of `dy`.
Thank you for your attention and support :)

Start by running the Rust reproduction with the supplied SVG and compare the flattened output for RTL and LTR text. Trace usvg's text layout and positioning for per- dy offsets, then verify that RTL lines retain separate baselines while glyph shaping remains unchanged.

Written by the indexing model from the issue text.