Condense an amended squash into its own checkpoint

Amending a squash after its first post-commit left every trailer with a checkpoint and no inherited-trailer marker, so post-commit found no target and skipped condensation, where a lone-trailer amend re-condenses into the session's own checkpoint. With every trailer existing, the last one, where prepare stamps, is now the preexisting target; stampedByAnotherCommit still lets only the session amending it write there.