chore: Remove deprecated integration tests, add modern samples, update snippets, CI, and docs (#4319)

## What this means for you

Most of this PR is behind-the-scenes cleanup, but there are a few things
that directly benefit you:

**Updated code snippets** -- The `ruiprop` snippet (available in Visual
Studio, Rider, and ReSharper) now generates the modern C# 13 `field`
keyword pattern instead of explicit backing fields:

```csharp
// Old snippet output
private string _name;
public string Name
{
    get => _name;
    set => this.RaiseAndSetIfChanged(ref _name, value);
}

// New snippet output
public string Name
{
    get;
    set => this.RaiseAndSetIfChanged(ref field, value);
}
```

**New sample applications** -- Three self-contained login samples you
can use as a starting point or reference for your own projects:

| Sample | Platform | What it demonstrates |
|--------|----------|---------------------|
| [ReactiveUI.Samples.Wpf](src/examples/ReactiveUI.Samples.Wpf) | WPF |
ReactiveUserControl, PasswordBox event marshaling |
|
[ReactiveUI.Samples.Winforms](src/examples/ReactiveUI.Samples.Winforms)
| WinForms | IViewFor with programmatic UI layout |
| [ReactiveUI.Samples.Maui](src/examples/ReactiveUI.Samples.Maui) | MAUI
| Shell navigation, ReactiveContentPage, DisplayAlertAsync |

Each sample shows RxAppBuilder initialization, WhenActivated lifecycle,
Bind/BindCommand, reactive validation (login button disabled until
fields are filled), async command execution with cancellation, and
proper subscription disposal.

The existing [Builder.WpfApp](src/examples/ReactiveUI.Builder.WpfApp)
and [Builder.BlazorServer](src/examples/ReactiveUI.Builder.BlazorServer)
examples have also been updated to use the `field` keyword.

**Visual Studio for Mac snippets removed** -- The platform has been
discontinued by Microsoft. Rider snippets remain available for macOS
users.

**Updated README** -- Examples section added with links to all sample
projects. UWP and Tizen removed from the packages table. MAUI section
updated with supported .NET 9/10 TFMs.

**Updated project docs** -- Code of Conduct now references the .NET
Foundation Code of Conduct directly. Contributing guidelines simplified
to point to the official docs at reactiveui.net/contribute.

## Behind the scenes

- Removed the `integrationtests/` directory (UWP,
Xamarin.iOS/Android/Mac, net472 WPF/WinForms) -- all targeted deprecated
platforms
- Added CodeQL security scanning via shared `actions-common` reusable
workflow (C#, Actions, JavaScript)
- Removed obsolete `build-samples.yml` workflow
- MAUI sample builds as net10.0 library on CI to work around platform
SDK issues

---------

Co-authored-by: Copilot <[email protected]>

Author: glennawatson

.gitattributes

@@ -1,289 +1,63 @@
-# Catch all for anything we forgot. Add rules if you get CRLF to LF warnings.
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+# Binary files (do not normalise or diff)
+*.png      binary
+*.jpg      binary
+*.gif      binary
+*.ico      binary
+*.pfx      binary
+*.snk      binary
+
+# Git LFS - images
+*.png      filter=lfs diff=lfs merge=lfs -text
+*.jpg      filter=lfs diff=lfs merge=lfs -text
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+
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+
+# Git LFS - archives
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+*.7z       filter=lfs diff=lfs merge=lfs -text
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+
+# Git LFS - compiled binaries
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+*.exe      filter=lfs diff=lfs merge=lfs -text
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+
+# Git LFS - documents
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