fix(build)!: honor consumer implicit usings

[glennawatson]

Summary

Respect the consuming project's ImplicitUsings setting in the package imports.

• Gate the lean, Reactive and WPF transitive namespace imports on true or enable.
• No test file is added. MSBuild evaluation of each package props file and a consuming project checks the package behavior directly.

Why

Projects with <ImplicitUsings>false</ImplicitUsings> receive ReactiveUI global usings from the packages. Closes #4535.

Breaking changes

Projects that disable implicit usings must add explicit using directives for ReactiveUI binding namespaces they use.

How this was verified

MSBuild evaluation found zero package usings with false and the expected package usings with true and enable for all four props files. Both core flavors built on net10.0 and net11.0 with zero warnings. The net10.0 ReactiveUI and ReactiveUI.Reactive test projects passed (880 and 882 tests).

Notes for the reviewer

The WPF props files were checked through MSBuild item evaluation; WPF assemblies were not built on Linux.

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

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% 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 93.75%. Comparing base (8831268) to head (92c74ea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4539   +/-   ##
=======================================
  Coverage   93.75%   93.75%           
=======================================
  Files         175      175           
  Lines        6182     6182           
  Branches      736      736           
=======================================
  Hits         5796     5796           
  Misses        386      386           

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