PacketSendObserver

Lets the consuming plugin observe packets that are sent directly (via connection.send), outside of any AbstractPacketBundle. The FMM packet sampler registers an observer so its per-tick report includes these unbundled sends (hitbox teleports, head rotations, mounts, etc.) that the bundle-wrapping sampler can't see.

No-op unless an observer is registered. The observer itself is expected to be cheap and to ignore sends when it isn't actively sampling.