Some characters display as boxes with Text2d

## Bevy version and features

Bevy 0.18 or 0.19.0-rc.2, default features plus `experimental_bevy_feathers` or `bevy_feathers`

## \[Optional\] Relevant system information

cargo 1.95.0 (f2d3ce0bd 2026-03-21), Ubuntu 25.10

## What you did

Run the following code:
```rust
use bevy::prelude::*;
use bevy::feathers::*;

fn main() {
App::new()
.add_plugins(DefaultPlugins)
.add_plugins(FeathersPlugins)
.add_systems(Startup, startup)
.run();
}

fn startup(mut commands: Commands) {
commands.spawn(Camera2d);
commands.spawn(Text2d::new("Ω"));
// Uncommenting the line below causes the "Ω" above to be rendered correctly
// commands.spawn(controls::radio((), Spawn((Text::new(format!("Test")), theme::ThemedText))));
}
```

## What went wrong

I expected the code to display a `Ω` character, but it instead displays as a box.

## Additional information
In bevy 0.18, uncommenting the last line of `startup` causes the `Ω` to display correctly. I have no idea why. In bevy 0.19.0-rc.2, adding the equivalent of that line does *not* cause it to display correctly.