NuGet: package metadata + dotnet pack

[benaadams]

Problem

version.props and Directory.Build.props exist but there are no published NuGet packages and no packaging workflow.

Sub-tasks

• Consistent <PackageId>, <Description>, <Authors>, <PackageLicenseExpression>, <RepositoryUrl>, <PackageIcon> across all public .csproj files
• Verify dotnet pack produces valid .nupkg for each project
• <IsPackable>false</IsPackable> on test/sample/benchmark projects

Dependencies

Depends on: #99 (CI to validate packaging)
Unlocks: #138 (Source Link + XML docs)

Phase

Phase 4 (Polish & Release Readiness)