adnl: recent_inbound_peers inserted before signature verification (hardening)

[Perruer]

Summary

In adnl/adnl-local-id.cpp:87, recent_inbound_peers.insert(packet.from_short())
executes before signature verification at adnl-peer.cpp:265.

An attacker can fill the 60-slot set with forged from_short() values,
blocking new peers from their source IP for 60 seconds.

Suggested fix

Move insert() to after signature verification, or validate
from_short() against the verified sender identity.

Affected commit

a6298cf

[mouyd906]