An oversized postcode becomes its own batch 🟩

Behavior emerged from the packing rule — pinned straight to green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/backend/app/modelling/test_batching.py

@@ -28,3 +28,16 @@ def test_postcodes_are_never_split_across_batches() -> None:
             in_batch = [p for p in batch if p.postcode == postcode]
             everywhere = [p for p in properties if p.postcode == postcode]
             assert len(in_batch) == len(everywhere)  # whole postcode, one batch
+
+
+def test_an_oversized_postcode_becomes_its_own_batch() -> None:
+    # arrange — one postcode alone exceeds the cap; neighbours are unaffected
+    properties = _properties("B93 8SU", 60, start_id=0) + _properties(
+        "M20 4TF", 20, start_id=100
+    )
+
+    # act
+    batches = pack_postcode_batches(properties, batch_size=50)
+
+    # assert
+    assert [len(b) for b in batches] == [60, 20]