v0.6.18

perf: build the Auto page only while it's the visible tab

Like the Debug views, the Auto builder ingested every decoded frame regardless
of whether the Auto tab was showing — a per-frame UI hop for a view nobody was
looking at, which is wasteful on a busy bus. Gate it behind _auto_active
(tracked via TabActivated); the capped DebugBuffer keeps recording while hidden
and the Auto boxes are rebuilt from it when the tab is opened.

Bumps to 0.6.18.