Transformed UI node shows a one-pixel seam around an exact-fit child

## Bevy version and features

Bevy `main` at `0de26631b0603acdc945aeae5e05b07ce58bc4dc`, using default features.

## Relevant system information

Linux (Arch Linux), Vulkan.

```text
AdapterInfo { name: "AMD Radeon RX 7800 XT (RADV NAVI32)", vendor: 4098, device: 29822, device_type: DiscreteGpu, driver: "radv", driver_info: "Mesa 26.2.1-arch1.1", backend: Vulkan }
```

## What you did

A solid child exactly fills its parent. Scaling the parent by `1.05` exposes a seam around the child.

Save this as `ui_seam.rs`:

```rust
---cargo
[package]
edition = "2024"

[dependencies]
bevy = { git = "https://github.com/bevyengine/bevy", rev = "0de26631b0603acdc945aeae5e05b07ce58bc4dc" }
---

use bevy::prelude::*;

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

fn setup(mut commands: Commands) {
commands.spawn(Camera2d);
commands
.spawn(Node {
width: percent(100),
height: percent(100),
justify_content: JustifyContent::Center,
align_items: AlignItems::Center,
..default()
})
.with_child((
Node {
width: px(260),
height: px(340),
..default()
},
UiTransform::from_scale(Vec2::splat(1.05)),
BackgroundColor(Color::srgb_u8(155, 174, 193)),
children![(
Node {
width: percent(100),
height: percent(100),
..default()
},
BackgroundColor(Color::srgb_u8(20, 38, 58)),
)],
));
}
```

Run it with:

```sh
cargo +nightly -Zscript ui_seam.rs
```

## What went wrong

Expected: the dark child completely covers its parent.

Actual: the lighter parent color remains visible as a one-pixel seam around the child.

## Screenshot

Start by running the provided ui_seam.rs reproduction with cargo +nightly -Zscript and inspect Bevy's UI transform and rendering paths related to transformed nodes. Done means the exact-fit dark child fully covers the scaled parent without a visible one-pixel seam on the reported setup.

Written by the indexing model from the issue text.