Keep empty floppy drives in no-media state

[LinuxJedi]

Summary

• initialize connected empty floppy mechanisms with /CHNG asserted
• keep empty-drive step polling from clearing the media-change latch
• add internal/external empty-drive regressions and document the hardware behavior

Testing

• cargo test disk_change --lib
• cargo test --lib
• cargo fmt --check
• cargo clippy --all-targets --all-features --locked -- -D warnings
• git diff --check

Closes #119