Changes to events are not being saved to XML on save_all()

```py
for vol in anthology.volumes():
if vol.year in ('2018','2019') and "workshop" in str(vol.title).lower() and "student research workshop" not in str(vol.title).lower():
venues = vol.venues()
venue_ids = list(vol.venue_ids)
for v in venues:
if v.type!="workshop":
event = anthology.get_event(f'{v.acronym.lower()}-{vol.year}')
event.add_colocated(vol)
venue_ids.remove(v.id)
vol.venue_ids = tuple(venue_ids)
anthology.save_all()
```

Result: The venue tags are removed but colocated volume info is not added in the XML.

Reproduce with the provided script and verify that `anthology.save_all()` updates XML by removing venue tags but not colocated entries. Start by tracing `save_all()` and the serialization code path it uses, then inspect how `event.add_colocated(vol)` and `vol.venue_ids` are persisted for XML output. Focus on event/volume export logic to find where colocated links are dropped. Done when a fresh XML save includes colocated volume information for the modified events.

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