Two Pike cleanups.
--auto-split was a got-only third connection-count policy (beside -x and -s)
that invented rather than matched aria2. Removing it also retires the now-dead
autoConns/maxAutoConns and the per-host transport cap that existed only to scale
for it; min(split, M*-x) is the sole policy again.
main's exit-code mapping fell back to substring-matching third-party error text
(strings.Contains "connection refused"/"timeout"/...), which rots when a
dependency rewords a message. The typed checks (errors.Is on the syscall errno,
*net.DNSError, net.Error.Timeout, context.DeadlineExceeded) already cover the
real stdlib errors -- verified end to end: refused -> 6, bad host -> 19. The two
cases that genuinely needed the text match are our own errors, now typed
sentinels: httpdl.ErrTimeout (idle timeout) and the bt metadata timeout wrapping
context.DeadlineExceeded.
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 (got-only, opt-in): derive the connection count from file
size (one per min-split-size, up to 16), overriding -x/-s. default stays
aria2-faithful at 1 connection for a single server.
