core v0.6.0

Traffic statistics and higher bandwidth limits (new features, so a minor release). No breaking changes: game repos
update without code changes; the new interface call is optional. No wire protocol change.

• Traffic statistics (docs/protocol.md "Traffic statistics"): Session::Traffic() counts messages and bytes per app
  message id (sent per destination, received, relayed, refused sends); PeerStats::reliableQueued (UDP) is the
  reliable backlog; net::NetMonitor turns them, the transport stats and the rate-limit drops into rates. The dll
  logs a report every net_log_interval s (default 5) and shows the last second in the debug status text (F7).
  Optional GameModule::MessageName(id) names the game's messages there.
• Higher limits: the host's relay limit per guest is 1 MB/s with a 2 MB burst (was 64 KB/s, 128 KB), set from
  net_budget_kbps (KiB/s, default 1024); Coop::NetBudgetBytesPerS() hands games the same number. The interest
  manager's default budget is 512 KB/s per peer with a 0.25 s burst (was 64 KB/s, 0.1 s).
• Tests: net_loopback "throughput" (a guest streaming ~450 KB/s through the host, unreliable states at 60 Hz plus a
  47 KB reliable message twice a second: nothing dropped by the relay limit, every reliable message delivered,
  counters and monitor agree); the rate-limit test pins the old small limits; net_interest "default budget" (a busy
  co-op scene, ~175 KB/s for one peer, nothing deferred).