arena: silence crouch-button debug spam, accurate welcome log
- Remove per-frame console.log that fired on every paint of the mobile
crouch button (left over from earlier UI debug).
- Welcome log now prints actual peer handles (or "arena empty") instead
of the misleading "N already in" that was counting self.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>