The UserDatabaseSynchronizer outlives the scenes that use it, which made
the periodic background synchronization fragile in two ways:
1. A freed loading popup permanently broke the sync.
The teacher settings screen injects its LoadingPopup into the
synchronizer but never cleared the reference on scene exit. Since a
freed Object is not equal to null in Godot 4, the next timer-driven
synchronization called into the freed node, crashed the coroutine
after `synchronizing` had been set to true, and every subsequent
synchronize() call was then silently cancelled by the re-entrancy
guard until the app restarted.
- All popup guards now use is_instance_valid() (plus is_inside_tree()
where get_tree() is needed).
- The teacher settings screen clears the synchronizer's popup
reference in _exit_tree().
2. Server payloads were applied without shape validation.
_apply_server_response() indexed JSON fields (lesson unlock data,
remediation score tuples, confusion matrix entries) without checking
their types, as flagged by the five "TODO ADD SECURITY" comments. A
single malformed or legacy record raised a script error mid-sync,
which also wedged the `synchronizing` flag as described above.
- Lesson unlock entries, remediation score pairs and confusion matrix
entries are now validated before use; malformed records are logged
and skipped individually so one bad record cannot abort the whole
synchronization or corrupt local student data.
- The three copy-pasted remediation parsing blocks are factored into
a single _parse_score_remediation() helper.
Verified with the GUT test suite (58/58 passing).
Adrien Ufferte
c77e9b90ec