Add NuGet release-candidate runbook

[Courtland9777]

Summary

• add final NuGet release-candidate review evidence for IntervalTree.Net 0.1.0-alpha.1
• add a guarded manual publication runbook that explicitly does not approve publication
• link the release-candidate review and runbook from NuGet readiness docs and README

Validation

• curl -fsS https://api.nuget.org/v3-flatcontainer/intervaltree.net/index.json returned HTTP 404 via curl exit 22; status-only confirmation returned 404
• 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-pr3
• bash scripts/smoke-test-local-package.sh
• package artifact inspection confirmed nupkg metadata, README, XML docs, license expression, repository metadata, empty net8.0 dependency group, snupkg PDB content, and SourceLink map

Boundaries

• does not publish NuGet package or symbols
• does not create GitHub release/tag, secrets, API keys, package upload, or publish workflow
• does not approve publication or career-material claims