core v0.8.0

Faster detection of a game that was killed, without dropping a player whose game is only busy; a key press for the
game. New features, so a minor release. No breaking changes; the wire protocol version stays 7 (the new UDP packet
type is ignored by older peers, which only refresh their timeout on it).

• UDP keepalive thread (UdpOptions::keepaliveThreadS, default 0.25 s): when Poll() has not run for twice that
  long (the thread that polls is stalled, e.g. a game loading a world), a thread of the transport sends an ALIVE
  packet (type 9, header only) to every connected peer, so a short timeout does not drop a busy player. It gives up
  after keepaliveMaxStallS (60 s) of stall: a hung process is dropped like a killed one. KeepalivesSent().
• The dll's timeouts are settings: net_timeout_s (UDP, default 4 s, was 10 s) and net_steam_timeout_s
  (Steam P2P, default 6 s, pings every 0.5 s; the Steam transport has no keepalive thread), both 2..60. A killed game
  is now noticed after about 4.5 s over UDP. Measured in Mafia DE: the frame (and so Poll()) never stalled more than
  0.2 s, also during loading screens.
• ui_input::PressGameKey(dik, holdMs): a key press the game itself sees (the keyboard's DirectInput state and
  buffered events), for prompts like a loading screen's "Enter: continue". Ignored while one of our panels captures
  input; a game reads DirectInput only while its window has focus.
• Tests: net_loopback "keepalive thread covers a stall" (5 s without Poll(), host timeout 2 s: still connected),
  "hung guest dropped after the stall cover" (1.5 s cover + 2 s timeout: dropped after ~3.3 s); the 10 s timeout
  test runs with the keepalive thread off (a killed process).