fix so clicking blue discard graphs links
fix most of the discards-not-being-ignored issues
move links_{from,to}_found to within find_links_*
WIP: about to replace 'nearest' with 'focus'
add SortedSet.has() method
eliminate .linked flag on nodes
minor cleanups
replace nearest with focus
stop setting node.fixed
move dragging init from mouseup to mousemove
minor cleanups