Add package consumer verification for packed artifacts

[Skymly]

Problem

CiPack verifies the nupkg structure, but it does not currently create a temporary consumer project that installs the packed package and validates runtime, analyzer, and source generator behavior from the package boundary.

Proposal

Add a follow-up package consumer verification step before stable package publishing.

Acceptance criteria

• A packed DesignPatterns nupkg can be referenced by a temporary consumer project.
• The consumer validates runtime API usage and source generator execution from the package.
• The verification is wired into an appropriate Nuke target without publishing NuGet packages.