Repository: flutter
Issue: `NavigationRail` does not follow Material specification
Labels: framework,f: material design,a: fidelity,P2,team-design,triaged-design

### Steps to reproduce

There are two main issues, both of which are related.

---

Create a `NavigationRail` where the `alignment` is set to 0, with a couple of destinations. Create a non-zero height widget and use it as the `leading`.

Note the destinations are centered in the space remaining after `leading` takes its space.

![image](https://github.com/user-attachments/assets/fb8cf618-b45f-4688-9662-ca59eac107fd)

This occurs because of https://github.com/flutter/flutter/blob/a6c057c406103d68ecaa42da86063f533d4f7f72/packages/flutter/lib/src/material/navigation_rail.dart#L466-L476. A more advanced implementation would be required to center the destinations independently, but also not overlap with any `leading` (and assume an alignment of -1) when there are a larger number of destinations. I would be happy to contribute to fix this, but have no idea how to start this sort of layout. Maybe a dedicated `RenderObject` is necessary, as I would have thought the height of the destinations would need to be known?

Additionally, the spec only makes provisions for top, center, and bottom alignment. Flutter is much more flexible - this is probably fine, but just another thing to point out.

---

Add a `trailing` to the rail. It is not bottom aligned, instead it sits after the destinations.

![image](https://github.com/user-attachments/assets/fbd0a453-0d19-4a6c-a1ea-31269a215b76)

Interestingly, the Material spec makes no mention of a trailing widget. But the official Material site uses one :D. (The spec also says never to use destinations without labels visible, but the demo screen ignores this as well.)


### Expected results

![image](https://github.com/user-attachments/assets/4a67b223-230a-4f15-9805-7a003872420c)


### Actual results

See screenshots above.

### Code sample

Pop a `Row` with one child a `NavigationRail` into a Scaffold`. Add destinations, alignment, and leading/trailing as necessary.
I'm not adding an MRE as the offending code has been found.

### Screenshots or Video

_No response_

### Logs

_No response_

### Flutter Doctor output

<details open><summary>Doctor output</summary>

```console
[✓] Flutter (Channel beta, 3.28.0-0.1.pre, on Microsoft Windows [Version 10.0.22635.4660], locale en-GB)
    • Flutter version 3.28.0-0.1.pre on channel beta at C:\Users\lukas\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 3e493a3e4d (4 weeks ago), 2024-12-12 05:59:24 +0900
    • Engine revision 2ba456fd7f
    • Dart version 3.7.0 (build 3.7.0-209.1.beta)
    • DevTools version 2.41.0

[✓] Windows Version (11 Home 64-bit, 23H2, 2009)

[✓] Android toolchain - develop for Android devices (Android SDK version 36.0.0-rc3)
    • Android SDK at C:\Users\lukas\AppData\Local\Android\sdk
    • Platform android-35, build-tools 36.0.0-rc3
    • Java binary at: C:\Program Files\Android\Android Studio\jbr\bin\java
      This is the JDK bundled with the latest Android Studio installation on this machine.
      To manually set the JDK path, use: `flutter config --jdk-dir="path/to/jdk"`.
    • Java version OpenJDK Runtime Environment (build 21.0.3+-12282718-b509.11)
    • All Android licenses accepted.

[✓] Chrome - develop for the web
    • CHROME_EXECUTABLE = C:\Program Files\Google\Chrome Dev\Application\chrome.exe

[✓] Visual Studio - develop Windows apps (Visual Studio Build Tools 2022 17.10.4)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2022\BuildTools
    • Visual Studio Build Tools 2022 version 17.10.35027.167
    • Windows 10 SDK version 10.0.22000.0

[✓] Android Studio (version 2024.2)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 21.0.3+-12282718-b509.11)

[✓] VS Code, 64-bit edition (version 1.96.2)
    • VS Code at C:\Program Files\Microsoft VS Code
    • Flutter extension version 3.102.0

[✓] Connected device (3 available)
    • Windows (desktop) • windows • windows-x64    • Microsoft Windows [Version 10.0.22635.4660]
    • Chrome (web)      • chrome  • web-javascript • unknown
    • Edge (web)        • edge    • web-javascript • Microsoft Edge 132.0.2957.26

[✓] Network resources
    • All expected network resources are available.

• No issues found!
```

</details>