Mirror WrapConn behavior: WrapUDPConn now increments total and current
connection counts on creation (one accepted UDP client session == one
connection) and decrements the current count on Close. Close is guarded
by a mutex and a closed channel so it is safe to call multiple times —
the current-conns decrement fires exactly once and the monotonic total
is untouched.
The WrapUDPConn constructor returns nil when either arg is nil, so
c.stats is always non-nil in all udpConn methods. Remove 8 redundant
nil guards that were inconsistent with packetConn (which already
skipped them).
Add 65 unit tests across plugin, stats, and wrapper packages covering
nil safety, race conditions, double-close, event marshaling, and
reset-traffic semantics. Replace magic numbers in tests with named
Kind constants.
ginuerzh