Implement destination-side ratchet lifecycle for SINGLE destinations

[doubleailes]

Summary

Destinations in Python Reticulum can enable ratchets, rotate them periodically, retain old keys, and optionally enforce ratchet usage. Rust currently lacks this lifecycle.

Affected code

• src/destination.rs
• SingleInputDestination

Required changes

• Add ratchet state:
  • retained ratchet private keys
  • rotation interval
  • retention limit
  • enforcement flag
• Implement:
  • enable_ratchets
  • rotate_ratchets_if_needed
  • setters for interval and retention

Acceptance criteria

• Ratchets rotate on schedule.
• Old ratchets are retained up to the limit.
• Enforcement flag affects decryption behavior.