The old PUNKBUSTER check compared client-reported position to server
position and snapped the player back when latency made them diverge,
which felt like getting teleported under any real network conditions.
Replaces that with proper client-side prediction + reconciliation:
client tags each input with a sequence number and keeps an input
buffer; server tracks the last processed sequence and reports its
authoritative position via a per-user inputAck alongside each
worldUpdate. The client only corrects when the actual disagreement
exceeds what the unacked input time can explain — so steady-state
movement runs purely on local physics, and only genuine unexpected
events (collisions, being hit) trigger a smooth blend toward the
server state.
Includes adaptive threshold scaling so high-latency sessions don't
false-positive corrections during normal running.
The state of the shift modifier is now distributed across the
network. Walking speeds and animation states are being updated
according to it.
Fixes#130
