Fix pooled packet reuse during concurrent ACK and resend

[NikoCat233]

A reliable packet could be acknowledged and recycled while a resend still referenced its buffer. A subsequent send could overwrite that buffer, causing the resend to transmit unrelated data.

This change:

• Acquires a buffer usage under the packet lifecycle lock before resending.
• Uses generation-tagged registrations to prevent stale references from resending or recycling a reused Packet.
• Releases buffer usages when asynchronous send setup or queueing fails, including DEBUG delayed sends.
• Keeps transport calls and application callbacks outside the lifecycle lock.

The wire protocol remains unchanged.

Validation:

• New regression tests: 11/11 passed in Release and 12/12 in Debug, including a controlled race using real UDP and ACK packets.
• Builds passed for net472 and netstandard2.0.
• Selected existing network suite: 100 passed, 1 failed, 2 skipped. The previously observed timing-sensitive UdpReliableMessageAckWithDropTest failed in the grouped run and passed in isolation; the grouped run is not considered fully passing.