v0.2.0

Changed

• Breaking: Unified UdpTransport and SharedUdpTransport into a single
  UdpTransport with UdpHandle pattern. The new design uses a background
  receiver task with sharded pending maps for correct concurrent request handling.
• Breaking: Transport trait changes:
  • Renamed target() to peer_addr() for consistency with std
  • Added register_request(request_id, timeout) for pre-send slot registration
  • recv() no longer takes a timeout parameter (uses registered deadline)
• Request IDs are now allocated from a global counter for process-wide uniqueness,
  preventing collisions when multiple transports exist

Added

• Configurable retry strategies with backoff support:
  • RetryStrategy::Fixed - constant delay between retries (default)
  • RetryStrategy::Exponential - exponential backoff with configurable base and max
  • RetryStrategy::None - disable retries entirely
• UdpTransport::shutdown() for graceful termination of background receiver
• ClientBuilder::build_with(&UdpTransport) for convenient shared transport usage

Fixed

• Concurrent UDP requests no longer cause false timeouts due to race conditions
  in response routing
• Memory leak from orphaned pending responses now prevented via periodic cleanup

Removed

• SharedUdpTransport and SharedUdpHandle (functionality merged into UdpTransport)