core v0.3.0

Kick and kick-and-ban (new features, so a minor release). No breaking changes: game repos update without code
changes; everything new in the interfaces is additive. The wire protocol changes (kProtocolVersion 6), so players
need the same build anyway.

• Session: host-only Kick(id, reason) and KickAndBan(id, reason). The kicked guest gets a reliable Kicked
  message (0x0C) with the reason and is dropped after the same 1 s linger as a Reject; its onDisconnected gets
  "You were kicked by the host[: reason]" (WasKicked() / WasBanned()), the other guests get PeerLeft "kicked by
  the host". Guests cannot kick (the API refuses, a Kicked sent to the host is ignored).
• Ban list for the hosted session (BanList(), Unban(index), ClearBans()), keyed by the peer's address (UDP IP
  without the port) and its SteamID (new optional Transport::PeerSteamId, implemented by the Steam and multi
  transports). A banned Hello gets Reject 7 "You are banned from this session" (RejectReason::Banned). Leave()
  and every Host() clear the list.
• Co-op UI: in the pause CO-OP panel the host can select a guest in the player list (keyboard up/down, mouse,
  gamepad) and choose KICK or KICK AND BAN, confirmed with "Kick ? Enter = yes, Esc = no". Guests see the
  banner "You were kicked by the host" / "You are banned from this session" and go back to idle.
  Coop::Kick, Coop::BanList, Coop::Unban; Coop::PlayerRow::id. No new MenuCommand.
• Tests: new net_kick (reason delivered under loss, PeerLeft to the others, guests cannot kick, ban by address and
  by SteamID through a test transport, unban, ban list cleared by a new session, kicked-not-banned rejoin).