XAML SG could warn when binding to a readonly property

### Description

For example:
```xml

```

Is a valid XAML, but at runtime the height is not applied, and it produces a warning log message:
```
Microsoft.Maui.Controls.BindableObject: Warning: Cannot set the BindableProperty "Height" because it is readonly.
```

Unfortunately, the message is not very actionable as it doesn't say on which element the property is being set and/or where it's set from. It' possible to diagnose this by plugging in a custom logger and setting a breakpoint into it, but that's pretty tedious.

Also - it's pretty easy to miss this log message completely and not know that the value is not applied.

With XAML SG, we generate the call to `SetValue` at compile time, so in theory we might be able to detect read-only properties and produce a warning at compile time.

### Public API Changes

N/A

### Intended Use-Case

Detect XAML which will not do what it "seemingly" does.
Ran into this with AI generated XAML. IntelliSense seems to know about it, and it doesn't offer `Height` as a property to be used in XAML, but it's still easy to get this wrong.

The issue names no files, tests, or entry points. Start by locating the XAML source-generator code that emits SetValue for bindings, then inspect how readonly BindableProperty values are represented. Done means the sample produces a compile-time warning that identifies the affected element and property.

Written by the indexing model from the issue text.