Add NuGet Trusted Publishing workflow

[Courtland9777]

Summary

• Add a manual-only NuGet Trusted Publishing workflow for IntervalTree.Net 0.1.0-alpha.1.
• Add a package artifact inspection helper used by the workflow and local validation.
• Update NuGet readiness/runbook docs to replace the manual API-key path with Trusted Publishing policy setup and post-attempt source-of-truth follow-up.

Validation

• git diff --check
• dotnet format IntervalTree.Net.sln --verify-no-changes
• dotnet restore IntervalTree.Net.sln
• dotnet build IntervalTree.Net.sln --configuration Release --no-restore
• dotnet test IntervalTree.Net.sln --configuration Release --no-build
• dotnet pack src/IntervalTree.Net/IntervalTree.Net.csproj --configuration Release --no-build --output artifacts/package-workflow-review
• bash scripts/smoke-test-local-package.sh
• bash scripts/inspect-package-artifacts.sh artifacts/package-workflow-review
• `python3 -c 'import yaml, pathlib; yaml.safe_load(pathlib.Path(".github/workflows/nuget-publish.yml").read_text()); print("YAML syntax OK")'