Test coverage: SnapshotWorker reference removal and cascade delete

[hahn-kev-bot]

Follow-up from the test-suite review (batches merged as #92–#96). This is the most intricate merge logic in the engine and currently has no coverage.

Gap

SnapshotWorker.MarkDeleted: when an entity is deleted, references to it are removed from referencing snapshots (RemoveReference), and if that removal itself deletes another entity the method recurses (deletedByRemoveRef → recursive MarkDeleted). The recursion guard comment (L142–146) implies past loop bugs.

harmony/src/SIL.Harmony/SnapshotWorker.cs

Lines 124 to 148 in 03f609f

	private async ValueTask MarkDeleted(Guid deletedEntityId, ChangeContext context)
	{
	// Including deleted shouldn't be necessary, because change objects are responsible for not adding references to deleted entities.
	// But maybe it's a good fallback.
	var toRemoveRefFrom = await GetSnapshotsReferencing(deletedEntityId, true)
	.ToArrayAsync();
	var commit = context.Commit;
	foreach (var snapshot in toRemoveRefFrom)
	{
	var updatedEntry = snapshot.Entity.Copy();
	var wasDeleted = updatedEntry.DeletedAt.HasValue;
	updatedEntry.RemoveReference(deletedEntityId, commit);
	var deletedByRemoveRef = !wasDeleted && updatedEntry.DeletedAt.HasValue;
	await GenerateSnapshotForEntity(updatedEntry, snapshot, context);
	//we need to do this after we add the snapshot above otherwise we might get stuck in a loop of deletions
	if (deletedByRemoveRef)
	{
	await MarkDeleted(updatedEntry.Id, context);
	}
	}
	}

No test:

• deletes a referenced entity (e.g. a Tag referenced by a Word/WordTag) and asserts the reference is scrubbed from the referrer;
• exercises the cascade where removing a reference deletes the referrer, driving the recursion.

Suggested tests

• DeletingReferencedEntityRemovesReferenceFromReferrers
• DeletingEntityCascadesWhenRemovalDeletesReferrer

Why it matters

Reference scrubbing and cascade deletion are core CRDT-merge behaviors; a regression here silently corrupts entity relationships. The recursion guard is untested against the loop it exists to prevent.