Enforce that spawned asyncio tasks are kept referenced

RUF006 catches a create_task whose result is discarded. The loop holds only a
weak reference, so such a task can be collected while still pending and the
work disappears without a trace.

Selected last, once the three existing offenders are gone, so the tree stays
clean under it from this commit on.

ruff.toml

@@ -76,6 +76,9 @@ select = [
     "B023",     # closure captures a loop variable, so every closure sees the
                 # last value rather than the one from its iteration
     "B035",     # dict comprehension with a static key
+    "RUF006",   # asyncio.create_task result discarded.  The loop keeps only a
+                # weak reference, so an unreferenced task can be collected
+                # while it is still pending, and the work is silently dropped
 ]
 
 # Deliberately not selected, though currently at zero: B905 (zip without an