[ScottishTapWater]

Summary

• Decompose archive verification into named identity, manifest, dependency and completeness checks.
• Separate the data-only package version map from its injected writer; leave execution-scoped generated imports in place without cleanup bookkeeping.
• Use asynchronous streams for version-map writes, archive verification and archive copying, with cancellation passed through the executor.
• Remove plan commit pinning and HEAD checks, allow external project paths, and overwrite generated archives with matching filenames.
• Update affected public API comments and tests, using TUnit's supplied token for the forwarding test.

Stacked on #23.

Verification

• Independent production and test reviews completed.
• Serial rebuild: zero warnings or errors.
• dotnet test src/PackageCascade.slnx --no-build --no-restore — 125 passed.
• dotnet format src/PackageCascade.slnx --verify-no-changes --no-restore.
• git diff --check.