[ScottishTapWater]

Summary

• Keep private project references in the graph used for build order, release propagation and referenced-package versions.
• Exclude evaluated PrivateAssets="all" references from the package dependencies expected in generated manifests.
• Add discovery and planner regressions plus two end-to-end tests of the same cascading private-reference release: streamed plan stdout into apply stdin, and plan --outfile followed by apply --infile.

Stacked on #25. Resolves the blocker found during the fresh whole-stack review without weakening archive verification.

Verification

• Full solution: 140 tests passed.
• dotnet format src/PackageCascade.slnx --verify-no-changes --no-restore.
• git diff --check.
• Independent production and test reviews passed; the original reviewer confirmed the blocker is resolved.