Fix concurrent Pipeline Cache publication correctness

[David Federman (dfederm)]

Summary

This fixes a race in concurrent Pipeline Cache publication where selector state could be read, merged, and rewritten inconsistently, causing one publisher to lose another publisher’s selector or output identity. The change makes selector publication immutable/serialized, includes the complete selector in output identity, isolates the seed-6 protocol from the legacy layout, and retries conflict-winner visibility when the exact entry is not immediately readable. It also corrects CAS diagnostics so a null AddOrGetContentHashListResult is treated as successful acceptance of the submitted value.

Why this is safe

• Scope is limited to concurrent publication and lookup paths; normal publish/restore behavior is preserved.
• The new seed-6 layout isolates incompatible selector/output keys from legacy writers, so old and new protocols do not mix.
• Conflict-winner retry only applies to required exact-key visibility; ordinary cache misses keep their existing behavior.
• Selector publication now relies on immutable predecessor/successor keys instead of a read/merge/write race.

Testing

Test / build	Result
dotnet build MSBuildCache.sln -c Release	pass
dotnet test MSBuildCache.sln -c Release --no-build	532/532 passed