Sub-Ladder rungs fill from the highest-yield segment first 🟥

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

tests/domain/modelling/test_solar_config_selection.py

@@ -256,3 +256,51 @@ def test_small_roof_derives_sub_ladder_rungs_below_googles_smallest_config() ->
     assert [c.panels_count for c in configs] == [2, 3]
     assert [c.yearly_energy_dc_kwh for c in configs] == [639.0, 958.5]
     assert [sum(s.panels_count for s in c.segments) for c in configs] == [2, 3]
+
+
+def test_sub_ladder_rungs_fill_from_the_highest_yield_segment_first() -> None:
+    # ADR-0058 — filling follows the ADR-0038 "by generation" precedent: the
+    # derived rung takes panels from the better-yielding plane first, each
+    # kept segment scaled pro-rata, never biased by segment tuple order.
+
+    # Arrange — smallest rung spans SE (2 panels, 250/panel) listed FIRST and
+    # SW (2 panels, 350/panel) listed second; max 5 → cap 3.5 → rungs 2 and 3.
+    south_east = SolarRoofSegment(
+        segment_index=0,
+        panels_count=2,
+        azimuth_degrees=135.0,
+        pitch_degrees=30.0,
+        yearly_energy_dc_kwh=500.0,
+    )
+    south_west = SolarRoofSegment(
+        segment_index=1,
+        panels_count=2,
+        azimuth_degrees=225.0,
+        pitch_degrees=30.0,
+        yearly_energy_dc_kwh=700.0,
+    )
+    potential = SolarPotential(
+        panel_capacity_watts=400.0,
+        max_array_panels_count=5,
+        configurations=(
+            SolarPanelConfiguration(
+                panels_count=4,
+                yearly_energy_dc_kwh=1200.0,
+                segments=(south_east, south_west),
+            ),
+        ),
+    )
+
+    # Act
+    configs = select_conservative_configs(potential)
+
+    # Assert — rung 2 is the SW plane alone (700); rung 3 adds one SE panel
+    # pro-rata (700 + 250 = 950).
+    assert [c.panels_count for c in configs] == [2, 3]
+    assert [c.yearly_energy_dc_kwh for c in configs] == [700.0, 950.0]
+    rung_two, rung_three = configs
+    assert [s.azimuth_degrees for s in rung_two.segments] == [225.0]
+    assert {(s.azimuth_degrees, s.panels_count) for s in rung_three.segments} == {
+        (225.0, 2),
+        (135.0, 1),
+    }