Add solution and CI package validation

[Courtland9777]

Summary

• Add an IntervalTree.Net solution containing the library and test projects.
• Add CI for pull requests and pushes to master that restores, builds, tests, and packs only.
• Add package metadata, MIT license expression, README packaging, XML documentation generation, and .NET build artifact ignores.
• Update local development docs to use the solution and local pack validation without publishing.

Validation

• git diff --check: passed
• dotnet format IntervalTree.Net.sln: passed
• dotnet restore IntervalTree.Net.sln: passed
• dotnet build IntervalTree.Net.sln --configuration Release --no-restore: passed, 0 warnings, 0 errors
• dotnet test IntervalTree.Net.sln --configuration Release --no-build: passed, 12 passed, 0 failed
• dotnet pack src/IntervalTree.Net/IntervalTree.Net.csproj --configuration Release --no-build: passed, created IntervalTree.Net.0.1.0.nupkg

Used temporary local cache paths for validation: NUGET_PACKAGES=/tmp/intervaltree-net-nuget-packages and NUGET_HTTP_CACHE_PATH=/tmp/intervaltree-net-nuget-http-cache.

Package inspection

• Confirmed README.md is included in the .nupkg.
• Confirmed lib/net8.0/IntervalTree.Net.xml is included for IntelliSense/package consumers.
• Confirmed nuspec metadata includes id IntervalTree.Net, version 0.1.0, authors, description, tags, repository URL, MIT license expression, readme, and net8.0 dependency group.

Boundaries

• No package publication.
• No release or tag creation.
• No secrets or API keys.
• No repository visibility changes.