fix: update packet evaluation metric

[regularizer]

Updated packet evaluation metric in doc_split module to cover single page documents in the packet.

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[adiadd]

Thanks for this @regularizer - would we be able to soundly address/change/adjust the failing test cases to be correct and aligned?

[adiadd]

logic change looks good but we're deleting the test for this code path without replacing it - can you swap in a test that asserts single-page groups now get 1.0 instead? something like:

   def test_single_page_groups_score_perfect(self):
       """Single-page groups should receive a perfect ordering score of 1.0."""
       data = [
           _page("invoice", "inv-01", 1, "invoice", "inv-01", 1),
           _page("form", "form-01", 2, "form", "form-01", 2),
       ]
       df = pd.DataFrame(data)
       scores = calculate_ordering_score_per_group(df)
       assert len(scores) == 2
       assert all(v == 1.0 for v in scores.values())
       assert calculate_average_ordering_score(scores) == 1.0

just wanna make sure the path we're changing stays covered. rest lgtm

[regularizer]

Sounds good. Updated.