CI: NuGet publish on tag

[benaadams]

Problem

No automated release pipeline exists. Tagged releases should publish NuGet packages.

Sub-tasks

• GitHub Actions workflow triggered on v* tags
• dotnet pack --configuration Release
• dotnet nuget push to NuGet.org (API key in repo secrets)
• Optional: pre-release packages on every merge to master (e.g., 1.0.0-preview.42)
• README badges: NuGet version, download count

Dependencies

Depends on: #138
Unlocks: public consumption of the library

Phase

Phase 4 (Polish & Release Readiness)