feat: follow WinForms host properties through ReactiveUI.Binding's ObservedProperty

[glennawatson]

Summary

With ReactiveUI.Binding 8.4.0 or later, the WinForms hosts follow their own properties through Binding's ObservedProperty, with WhenAnyValue semantics.

• The [RoutedControlHost] and [ViewModelControlHost] hosts use ObservedProperty when the project's ReactiveUI.Binding has it. Create replaces WhenAnyValue for DefaultContent, Content and ViewModel, and Then plus Switch replace WhenAnyObservable for Router.CurrentViewModel and ViewContractObservable, in the flavour the host's view API comes from.
• With an older ReactiveUI.Binding, or none, the hosts keep following their own PropertyChanged. A new ControlHostAnalyzer reports RXUISG0022 (Info) on each such host.
• No package dependency on ReactiveUI.Binding. The generator and the analyzer both decide from ViewApiRules, which looks for ObservedProperty in the referenced Binding flavour and now also picks the view API.

Why

Generated code has to keep WhenAny semantics without calling an API ReactiveUI.Binding intercepts.

• Binding's generator cannot see call sites in another generator's output, so the 4.0.1 hosts moved to their own PropertyChanged following; ReactiveUI.Binding 8.4.0 adds ObservedProperty, a non-intercepted, AOT-safe observation API with WhenAnyValue's semantics, for exactly this case.
• A package dependency would force one Binding flavour on every consumer, so the choice follows what the project references.

Breaking changes

None. Hosts on ReactiveUI.Binding 8.4.0 or later follow their properties through Binding's observation, including any registered observation provider; other hosts are generated as in 4.1.0.

How this was verified

New tests compile and run both hosts against the real ReactiveUI.Binding 8.4.0, lean and .Reactive, showing the default content and then the routed or hosted view; the analyzer is tested with and without ObservedProperty.

• Scratch apps on a ReactiveUI build that depends on ReactiveUI.Binding, in both flavours, run the hosts with Binding 8.4.0 (generated through ObservedProperty) and with 8.1.0 (the fallback).

Notes for the reviewer

Start with ViewApiRules.cs and the three Append* members in ControlHostExtensions.cs.

• ControlHostWriter is renamed ControlHostExtensions, as it now holds an extension block.
• The tests reference ReactiveUI.Binding for its runtime types only; the test project removes Binding's generator and analyzer from the compiler, which ExcludeAssets alone did not.
• No published ReactiveUI is built on ReactiveUI.Binding yet, so the host tests declare the few ReactiveUI types a host touches in source.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.75%. Comparing base (7158aa0) to head (344714a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #506      +/-   ##
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+ Coverage   96.70%   96.75%   +0.05%     
==========================================
  Files          74       76       +2     
  Lines        3759     3824      +65     
  Branches      531      544      +13     
==========================================
+ Hits         3635     3700      +65     
  Misses        124      124              

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