Leaf lines below this level are silently skipped: log_debug() counts as
"debug", log_info() and log_success() count as "info", log_warn()
as "warn", log_error() as "error". Step open/close lines always
render regardless of verbosity, since hiding them would break the tree
structure. Suppressed log_warn()/log_error() calls still elevate the
enclosing step's close glyph – verbosity only hides the leaf line's own text.
Usage
logtree_threshold(level = c("debug", "info", "warn", "error"))