Prepare NuGet prerelease metadata

[Courtland9777]

Summary

• set package metadata to prerelease candidate version 0.1.0-alpha.1
• add missing NuGet-facing metadata fields and release notes
• clarify README, changelog, and status docs that the package is not published
• add a concise NuGet readiness note with the exact package ID check result

Validation

• git diff --check
• dotnet format IntervalTree.Net.sln --verify-no-changes using temporary NuGet caches
• dotnet restore IntervalTree.Net.sln using temporary NuGet caches
• dotnet build IntervalTree.Net.sln --configuration Release --no-restore using temporary NuGet caches
• dotnet test IntervalTree.Net.sln --configuration Release --no-build using temporary NuGet caches
• dotnet pack src/IntervalTree.Net/IntervalTree.Net.csproj --configuration Release --no-build --output artifacts/package-pr1 using temporary NuGet caches
• inspected generated .nupkg metadata and contents, then removed package artifact

No NuGet publication, package upload, tag, release, publish workflow, API key, or secret changes were performed.