Add NuGet consumer smoke test

[Courtland9777]

Summary

• Enable GitHub SourceLink metadata and CI-friendly deterministic package settings for the library package.
• Enable .snupkg symbol package generation during dotnet pack.
• Add a local external-consumer smoke test script that packs the current version, creates a temporary /tmp consumer app, installs IntervalTree.Net from the local package source, and validates construction, adding intervals, point query, and overlap query behavior.
• Wire the smoke test into CI after pack and document the SourceLink, symbols, and local consumer validation status.

Validation

• git diff --check
• dotnet format IntervalTree.Net.sln --verify-no-changes
• NUGET_PACKAGES=/tmp/intervaltree-net-nuget-packages NUGET_HTTP_CACHE_PATH=/tmp/intervaltree-net-nuget-http-cache dotnet restore IntervalTree.Net.sln
• NUGET_PACKAGES=/tmp/intervaltree-net-nuget-packages NUGET_HTTP_CACHE_PATH=/tmp/intervaltree-net-nuget-http-cache dotnet build IntervalTree.Net.sln --configuration Release --no-restore
• NUGET_PACKAGES=/tmp/intervaltree-net-nuget-packages NUGET_HTTP_CACHE_PATH=/tmp/intervaltree-net-nuget-http-cache dotnet test IntervalTree.Net.sln --configuration Release --no-build
• NUGET_PACKAGES=/tmp/intervaltree-net-nuget-packages NUGET_HTTP_CACHE_PATH=/tmp/intervaltree-net-nuget-http-cache dotnet pack src/IntervalTree.Net/IntervalTree.Net.csproj --configuration Release --no-build --output artifacts/package-pr2
• NUGET_PACKAGES=/tmp/intervaltree-net-nuget-packages NUGET_HTTP_CACHE_PATH=/tmp/intervaltree-net-nuget-http-cache bash scripts/smoke-test-local-package.sh

Package Inspection

• Fresh pack produced IntervalTree.Net.0.1.0-alpha.1.nupkg and IntervalTree.Net.0.1.0-alpha.1.snupkg.
• .nupkg metadata includes ID/version/authors, MIT license expression, README, project URL, release notes, repository URL/type/commit, and an empty net8.0 dependency group.
• .nupkg includes README.md, lib/net8.0/IntervalTree.Net.dll, and lib/net8.0/IntervalTree.Net.xml.
• .snupkg contains lib/net8.0/IntervalTree.Net.pdb and SymbolsPackage metadata.
• PDB string inspection showed a SourceLink map to raw.githubusercontent.com/Courtland9777/IntervalTree.Net/<commit>/*.

No NuGet package or symbol package was published, no upload or release workflow was added, and no secrets, tags, or GitHub releases were created.