main_result called send_stats_report/calculate_stats with 'let _ = ...',
silently dropping any failure. A broken welist route (e.g. unreachable
IPv4 path) is then invisible in the logs and can go unnoticed for a long
time. Log failures with warn! so connectivity problems are diagnosable.
The welist host publishes both A and AAAA records. On networks where the
IPv4 route is broken (connection stalls after the TCP handshake) while
IPv6 works, the default connector may pick the broken family and the
report request hangs.
When BAL_PUSHER_PREFER_IPV6 is truthy, the pusher now resolves the welist
host itself and pins the reqwest client to its first IPv6 address; the
original hostname is still used for the Host header and TLS SNI. When the
variable is unset (default) or no AAAA record exists, behavior is
completely unchanged.
Includes unit tests for the URL host/port parsing and documentation in
docs/07_deployment_and_ops.md.
