… protocol API (#323)

* refactor(websocket): migrate TCP receive path to std::span and update protocol API

- Update websocket_protocol::process_data() to accept std::span<const uint8_t>
  instead of const std::vector<uint8_t>&
- Update websocket_socket to use tcp_socket::set_receive_callback_view() for
  zero-copy TCP-to-WebSocket data flow
- Change on_tcp_receive() to accept std::span<const uint8_t>
- Update all handshake callbacks to use span-based receive

This eliminates per-read std::vector allocation solely for TCP-to-protocol
handoff. Data is copied once into the protocol's internal buffer for frame
processing.

Part of Epic #315 (TCP receive zero-allocation hot path)
Closes #318

* docs: add WebSocket zero-copy receive path to changelogs

Document the WebSocket receive path migration to std::span callback:
- websocket_protocol::process_data() API change
- websocket_socket using tcp_socket::set_receive_callback_view()
- Performance improvement details

Part of Epic #315, #318

* fix(tcp_socket): add socket validity check before async read operation

Add is_open() check in do_read() to prevent UBSAN errors from accessing
null descriptor_state when the socket has already been closed.

This fixes BoundaryTest.HandlesSingleByteMessage failure under
undefined behavior sanitizer.

* docs: add tcp_socket UBSAN fix to changelogs

Document the socket validity check fix in all changelog files:
- Added is_open() check before async_read_some() in tcp_socket::do_read()
- Prevents null descriptor_state access when socket is already closed
- Fixes BoundaryTest.HandlesSingleByteMessage UBSAN failure