1.0: Make migrate finish and report its state correctly #1324

Goal: migrate names the right next step, survives transient failures, and never treats an unknown source as unmarked.

• major: migrate status next never moves on to the unmark. src/cmd/migrate/status.rs:136.
• major: A minidoc with no pds is reported as "not marked" and "nothing to do". src/cmd/migrate/mod.rs:81.
• minor: The 10k listing budget makes migrate fail for large collections. src/cmd/migrate/import.rs:329, 675, status.rs:159.
• minor: Every unmark failure exits 1. src/cmd/migrate/import.rs:727.
• minor: Blob uploads are not retried on 429. src/cmd/migrate/import.rs:215.
• minor: The import --json dry run uses the result shape when unmarked. src/cmd/migrate/import.rs:116.
• minor: --unmark help suggests a run that clap rejects. src/cmd/migrate/import.rs:33-38.