test(pkg): preserve per-platform alternative selections

[Alizter]

Summary

• Add a portable lock-directory regression for an unfiltered dependency alternative whose implementations are available on different platforms.
• Assert that locking preserves the implementation selected for each platform.
• Build the resulting lock directory on Linux and macOS to cover consumption as well as the lock summary.

This snapshots the existing per-platform selection invariant before the joint-solver change in #15982.

Tests

• nix develop -c dune runtest test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-platform-alternative-selection.t
• CI=true nix develop -c dune build @fmt @check