PacketSizeEstimator

Bridge that lets the version-agnostic consumer (e.g. FreeMinecraftModels' packet sampler) ask the active NMS module for the REAL serialized wire size of a packet, without the consumer needing any NMS imports.

The NMS module registers a PacketSizeFunction at adapter init via setImpl. Callers use sizeOf; if no implementation is registered (or it fails) they get -1 and should fall back to their own estimate.