fix(blazor): migrate to ReactiveUI.Primitives.Blazor

[ChrisPulman]

What kind of change does this PR introduce?

This is a Blazor scheduler and dependency compatibility fix. It migrates the Blazor packages from Reactive.Wasm to ReactiveUI.Primitives.Blazor and its System.Reactive variant, aligns required servicing dependencies, and adds lean/reactive regression coverage plus a runnable WebAssembly example.

closes #4430

What is the new behavior?

• Blazor WebAssembly uses WasmSequencer.Default in the lean package and WasmScheduler.Default in the System.Reactive package.
• Blazor Server retains the current-thread fallback; renderer-affine work remains component-scoped through ComponentBase.InvokeAsync, avoiding an invalid process-wide renderer dispatcher.
• WithBlazorWasm() consistently configures core services, the WASM main scheduler, the task-pool scheduler, and Blazor platform registrations.
• ASP.NET, Microsoft.Extensions.*, System.Text.Json, and WebAssembly package versions satisfy the ReactiveUI.Primitives 7.2.0 servicing floors.
• A Blazor WebAssembly scheduler-probe example and reactive Blazor test project validate both package seams.

What is the current behavior?

The Blazor projects depend on the removed Reactive.Wasm package and do not provide equivalent lean/reactive validation. After updating to ReactiveUI.Primitives 7.2.0, lower central ASP.NET, System.Collections.Immutable, and System.Text.Json pins cause package-downgrade restore failures.

Checklist

• 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

Root-cause fixes were made without adding warning suppressions, NoWarn, pragmas, warning demotion, or analyzer exclusions.

Validation completed:

• Full solution restore and build: 0 warnings, 0 errors
• ReactiveUI.Blazor.Tests: 38/38 passed
• ReactiveUI.Blazor.Tests.Reactive: 11/11 passed
• ReactiveUI.Splat.Tests: 6/6 passed
• ReactiveUI.Tests: 2,024/2,024 passed
• ReactiveUI.Tests.Reactive: 2,024/2,024 passed
• ReactiveUI.Builder.Tests: 187/187 passed
• BlazorReactiveUIBuilderExtensions: 100% line coverage in lean and reactive reports
• git diff --check: clean

[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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.92%. Comparing base (e227460) to head (0f1434d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4445      +/-   ##
==========================================
+ Coverage   93.89%   93.92%   +0.03%     
==========================================
  Files         359      359              
  Lines       15104    15105       +1     
  Branches     1568     1568              
==========================================
+ Hits        14182    14188       +6     
+ Misses        679      674       -5     
  Partials      243      243              

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