Add backend API tests for the two issue #265 features that shipped
untested: auto-positioning root nodes into a free grid slot (first at
origin, collision avoidance, child origin default, explicit coords and
explicit zero preserved) and auto-assigning edge handles from absolute
canvas Y (source above/below/equal target, partial handle fill, child
abs-Y resolved through parent).
Drop the max(0, round()) clamp in _find_free_position: it folded
negative-positioned nodes onto cell (0,0), falsely blocking or freeing
the origin slot. Negative nodes now keep their true cells and never
intersect the positive search space.
Pouzor
b7985306cd