max-connection-per-server, split and min-split-size were tagged HTTP, so
they only appeared under --help=all. Tag them Basic so they show in the
default --help, where users expect the core connection knobs.
an idle worker now steals the back half of the busiest in-flight segment
instead of exiting, so one slow segment no longer drains alone over a single
connection. control file persists stolen tails; resume re-tiles.
add --auto-split (opt-in): derive the connection count from file size (one
per min-split-size, up to 16), overriding -x/-s. default stays at 1
connection for a single server.
