Add PackagePtr::Start for FastTx package send

[NikolayChirkov]

Summary

Implements the FastTx step after EncodePacket: PackagePtr → Start().

• Ports the aether/prepared_packet/ foundation from prepared-packet-v0 / PR Add prepared send_message packet API #513 (PreparedSendMessageBlock, EncodePacket, PrepareSendMessage, export APIs).
• Adds Package / PackagePtr owning encoded bytes + destination endpoint.
• Package::Start(SendFunc) sends through a caller-provided raw send function (external code owns the UDP socket), with would-block retry via the task scheduler and WriteAction status events.
• Adds tests/test-prepared-packet covering success, retry, fail, and stop.

API sketch

auto block = prepared_packet::PrepareSendMessage(stream, nonce_count);
auto package = prepared_packet::MakePackage(ae_context, *block, payload);
package->Start([](auto const& endpoint, Span<uint8_t const> data)
                   -> std::optional<std::size_t> {
  // sendto(...) on externally owned socket
  return bytes_sent; // 0 = would-block, nullopt = fail
});

Test plan

• Configure + build aether and test-prepared-packet
• ctest . --progress -j -E "((sodium)|(hydro)|(bcrypt)).*" --output-on-failure — 23/23 passed
• tests/run/test-prepared-packet — 4/4 passed