Centralize stderr status rendering through core::Printer

[ryanfowler]

Summary

• Add shared core::Printer helpers for plain status lines and flushed stderr output
• Route update progress, update completion, retry messages, and the dry-run separator through the central printer path
• Add unit coverage for the new status helpers and the migrated retry/update rendering

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1