GitHub Actions CI for .NET 9

[unrealbg]

Summary

No automated CI for .NET 9, or not covering restore, build, test, and dotnet format.

Scope

• Area(s): Infrastructure, DevOps
• Non-goals: Manual build/test procedures

Plan

• Setup actions/setup-dotnet with .NET 9
• Cache NuGet; run dotnet format --verify-no-changes
• Run on push and PR to main/dev
• Add status badges to README

Acceptance criteria

• Runs on push and PR to main/dev
• Fails on test or formatting errors
• Status badges added to README
• Build, tests, and formatting pass in CI

Additional context

Labels: devops