Fix IndexError crash in work_process with multiple circular-descendant parents

[czxb7r]

Problem

Classical Extras crashes Picard (SIGABRT) while processing a release whose work hierarchy contains two or more circular parent references.

Reproduced with the work "Tannhäuser: Ouverture and Venusberg Music" (d801c361-fcfa-4485-81b4-486154daf6bd), which has two parents that are circular descendants. The plugin correctly logs Found parent which is descendant of child - not using, to prevent circular references (twice), then dies:

File "webservice/__init__.py", line 593, in _process_reply
File "webservice/__init__.py", line 578, in _handle_reply
File ".../classical_extras/__init__.py", line 4977, in work_process
IndexError: pop index out of range

Because this runs inside a QtNetwork reply slot, PyQt escalates the uncaught exception to qFatal() → abort(), taking the whole app down (no Python traceback shown in the GUI; just a crash).

Observed on installed 2.0.14 and present on this branch (2.0.11).

Root cause

In PartLevels.work_process, the circular-parent removal collects indices into the parallel parentIds/parents lists in del_list, then removes them with:

for i in list(set(del_list)):
    removed_id = parentIds.pop(i)
    removed_name = parents.pop(i)

set iteration order is arbitrary, and each pop(i) shrinks the list, so once a second index is popped a later index is out of range. A single circular parent happens to work by luck; two or more reliably crash.

Fix

Pop in descending index order so earlier removals don't invalidate later indices — one line:

for i in sorted(set(del_list), reverse=True):

Testing

With this change the same release tags through cleanly, the two circular parents are still correctly dropped and reported via ~cwp_error, and no crash occurs.

Environment: Picard 2.13.3, Python 3.11.5, PyQt 5.15.11, macOS.