Add RP0001 Roslyn analyzer for nullable reference type support

[Copilot]

Nullable reference types are enabled, but the compiler cannot flag the one case that produces a silent null: a ReactiveProperty-family value created for a non-nullable reference type without an initial value initializes Value to default(T) (i.e. null). Removing the historical default constructors would be a breaking change, so this is addressed with an analyzer — the approach converged on in the issue discussion.

Analyzer (RP0001, category Usage, Warning)

• New Source/ReactiveProperty.Analyzer project (netstandard2.0) with ReactivePropertyNullableAnalyzer.
• Flags creations where the type argument is a non-nullable reference type and no initial value is supplied:
  • new ReactiveProperty<T>(), new ReactivePropertySlim<T>(), new ReadOnlyReactivePropertySlim<T>(source)
  • source.ToReadOnlyReactivePropertySlim(), source.ToReadOnlyReactiveProperty()
• Ignores value types, nullable T?, open type parameters, and code with the nullable context disabled.
• For the ToReadOnly* extension methods, T is taken from the source IObservable<T> element type — the compiler infers those methods' return type as T? because of their initialValue = default parameter, so the return type can't be used.

Packaging

• Shipped inside both ReactiveProperty.Core and ReactiveProperty packages under analyzers/dotnet/cs (referenced with ReferenceOutputAssembly="false", so it adds no runtime dependency).

Tests & docs

• 12 MSTest cases via Microsoft.CodeAnalysis.CSharp.Analyzer.Testing.MSTest.
• Decision recorded in ADR 0006.

#nullable enable
var a = new ReactiveProperty<string>();               // RP0001
var b = new ReactivePropertySlim<string>();           // RP0001
var c = source.ToReadOnlyReactivePropertySlim();      // RP0001

var d = new ReactivePropertySlim<string>("initial");  // ok — initial value
var e = new ReactivePropertySlim<string?>();          // ok — nullable T
var f = new ReactivePropertySlim<int>();              // ok — value type

Out of scope

The ReactiveProperty (NETStandard) assembly has 91 pre-existing internal nullable warnings unrelated to this change; resolving them would touch public API nullability annotations (potentially breaking) and is left as separate follow-up work.