- pysimFsLoadChildren failure now only marks exists=false and re-renders;
loaded children stay in the node and are hidden by the renderer (red
cross, no toggle, no (empty)), so a failed DF is re-attempted on the
next probe/refresh instead of being short-circuited
- dropped the parent_sel-less /api/tree retry: error payloads now carry
exists:false, so it doubled requests for every absent DF and re-selected
the same FID without its parent (pySim probe_file fallback)
- probe walks strictly on exists===true and never collects files under a
non-existing DF; no longer clears children on a failed EF select
- tests updated: single request per failed load, children untouched,
200 exists:false marks absent without retry, render hides stale
children, probe never fetches/selects them; SW cache v116 -> v117.