Random I1 and MTU on install

I1 becomes a DNS query to a random popular domain (A/AAAA) instead of the
shared icloud.com response every key carried. The format sticks to tags both
the kernel module and amneziawg-go accept and is checked before awg1.conf is
written: a bad I1 fails the whole awg-quick up. With KEEP_DATA=y the old I1
is kept, or generated if missing — the server never checks it, old keys keep
their own.

MTU is picked from 1320–1376 (AWG_MTU overrides) and kept on reinstall.
awg-ctrl now reads MTU from awg1.conf for vpn:// keys instead of a constant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013bNGYiDst62wH3ZGTRPFLJ

internal/ctrl/awg.go

@@ -344,8 +344,10 @@ func (s *Server) syncPeers() error {
 // vpn:// собирались бы на старых параметрах — например, на 2.0 уже после перехода на 3.1.
 func (s *Server) reloadAwgParams() string {
 	p := ReadParams(s.confPath(), func(m string) { s.log.Warn(m) })
+	mtu := ReadMTU(s.confPath())
 	s.cfgMu.Lock()
 	s.c.Params = p
+	s.c.MTU = mtu
 	s.c.Gen = p.Gen()
 	s.c.Keepalive = keepaliveByGen[s.c.Gen]
 	g := s.c.Gen