core v0.2.0

Stability fixes plus a few small optional additions to the interface (so a minor release). No breaking changes: game repos update without code changes.

• Steam joins: an invite / "Join Game" request is only recorded in the Steam callback (which can run inside
  Session::Poll) and acted on at the start of the next Coop::Tick, so the session is never torn down while it is
  being polled (src/dll/pending_join.h).
• Crash dumps: only an exception that ends the process (unhandled filter, or heap corruption / stack buffer
  overrun) uses up the one-shot dump, and only once the dump was written; a caught first-chance stack overflow no
  longer prevents the dump of a later real crash. The dump is written from a writer thread started at install time;
  DllMain keeps 64 KB of stack back on every thread (>= 1 MB stacks) so an unhandled stack overflow still reaches
  the handler; on a thread without that reserve a stack overflow gives a crash-<time>-firstchance.dmp instead.
  The mod dll no longer calls DisableThreadLibraryCalls.
• Spam protection: the host rate-limits guest chat (5/s, burst 10) and the app messages it relays for a guest
  (64 KB/s, burst 128 KB); excess is dropped, counted (Session::RateLimits()) and logged at most once per second.
  New SessionConfig fields chatPerS, chatBurst, relayBytesPerS, relayBurstBytes. The UDP transport, when
  its incoming slots are full, evicts the oldest connection that has not completed the session handshake instead of
  refusing everyone; established players are never evicted (new optional Transport::MarkEstablished, called by
  the session; UdpTransport::Evictions()).
• Launcher: keys that need a value (--game, --join, --join-lobby, --log, --model, --name,
  --password, --port, --transport, --window, --window-timeout-s; table in src/launcher/args.h) refuse a
  missing, empty or flag-looking value (--password --name x) with an error (message box, or printed with
  --check-only / --yes) and exit 1 without starting the game. Bare flags stay bare.
• Tests: new pending_join, crash_dump, launcher_args; net_loopback (rate limits) and net_fuzz (eviction)
  extended. Wire protocol unchanged (kProtocolVersion 5).