fix(generator): declare observation helpers for every dispatch file

[glennawatson]

What kind of change does this PR introduce?

Bug fix (source generator emission and packaging).

What is the new behavior?

• The observation helper types a KVO or WinUI source needs (__KVOObserver, __KVOObservable<T> and the WinUI equivalents) are declared once, in a dedicated ObservationHelpers.g.cs, for any compilation that detects a type requiring them. Every dispatch file can now use them, so binding a KVO type through BindTwoWay, BindOneWay, Bind, OneWayBind, BindCommand, BindInteraction, WhenAny or WhenAnyObservable compiles.
• Declaration is driven by the detected type set rather than by call sites. A reference can only be emitted for a type the detection pipeline matched, so the declarations are provably a superset of the references and a future binding API cannot reintroduce the gap by forgetting to register itself.
• The runtime packages ReactiveUI.Binding and ReactiveUI.Binding.Reactive now carry the generator, so referencing the runtime package alone is enough. This is what the documentation already promised.

What is the current behavior?

Closes #48

• A compilation that used a KVO type only through a binding API, with no WhenChanged, WhenChanging or WhenAnyValue call anywhere, generated dispatch code referencing __KVOObservable<T> while nothing declared it. The build failed with CS0246: The type or namespace name '__KVOObservable<>' could not be found. Only the observation code generator declared the helpers.
• Separately, a compilation using two of WhenChanged, WhenChanging and WhenAnyValue on a KVO type declared the helpers more than once, because the observation generator runs once per API. That produced CS0102. This was latent and is fixed by the same change.
• Neither runtime package referenced the generator, so consumers had to add ReactiveUI.Binding.SourceGenerators by hand despite the documentation stating the generator is included automatically.

What might this PR break?

• ReactiveUI.Binding.SourceGenerators no longer carries the generator assemblies; it keeps only its props file and is now a compatibility package. This is deliberate: shipping the generator in both runtime packages while also shipping it here caused a consumer referencing more than one of them to load the generator twice, which failed the build with CS0111, CS0101 and CS0579. Existing consumers that reference the SourceGenerators package continue to build, because the generator arrives through the runtime package they already reference. Consider deprecating or unlisting that package separately.
• PlatformDetectionSnapshotTests.NSObject_Detected now uses the shared Apple platform stub and asserts the generated output compiles. Its previous minimal stub could snapshot helper code that does not compile.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

Verification:

• Generator tests cover an NSObject source used through BindOneWay and BindTwoWay. Both fail on main with the reporter's exact error and pass here.
• All three packages were packed and inspected, and three consumer projects were built against a local feed: runtime package alone (generator runs), both runtime packages together (single generation, clean build), and the reported shape of ReactiveUI.Binding.Reactive plus an NSObject source with BindTwoWay (clean build, helpers emitted).
• Build is clean with 0 warnings and 0 errors; the full suite passes 10764/10764.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.63%. Comparing base (c37f8ca) to head (f727861).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   97.61%   97.63%   +0.01%     
==========================================
  Files         227      228       +1     
  Lines        7752     7772      +20     
  Branches     1062     1066       +4     
==========================================
+ Hits         7567     7588      +21     
  Misses        143      143              
+ Partials       42       41       -1     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud