Complete the Nerdbank.GitVersioning transition — drop GitVersion.Tool

[ChrisonSimtian]

Per CLAUDE.md: "GitVersion is still installed as a transitional helper for MajorMinorPatchVersion in Build.cs; full removal is a follow-up."

This is that follow-up. Nerdbank.GitVersioning emits everything we need — AssemblyInformationalVersion, NuGetPackageVersion, etc. Build.cs should source MajorMinorPatchVersion from NB.GV's generated ThisAssembly.AssemblyInformationalVersion (or similar) and stop invoking the GitVersion CLI tool.

Refs the dependency audit. Done when:

• Build.cs MajorMinorPatchVersion derived from NB.GV
• GitVersion.Tool PackageDownload removed
• Test that this repo's release versioning still works (publish to nuget.org succeeds with a sensible version)

[ChrisonSimtian]

Closed in 737c7f9. GitVersion.Tool PackageDownload removed from build/_build.csproj; Build.cs's MajorMinorPatchVersion now reads from ThisAssembly.AssemblyInformationalVersion (NB.GV-generated, split off the +commitsha tail). NB.GV added as a direct PackageReference on _build.csproj since central package management is off there.

Fallout.Components.IHazGitVersion kept for downstream consumers — they can still use it if they have GitVersion CLI installed.