Sub-Ladder rungs never take from north planes 🟩

Pins behavior delivered with the tracer slice (derivation starts from the
north-dropped remainder) — no red phase; the mutation is discriminating
(a kept north panel would win fill-by-generation and shift the yields).

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

tests/domain/modelling/test_solar_config_selection.py

@@ -304,3 +304,47 @@ def test_sub_ladder_rungs_fill_from_the_highest_yield_segment_first() -> None:
         (225.0, 2),
         (135.0, 1),
     }
+
+
+def test_sub_ladder_rungs_never_take_from_north_planes() -> None:
+    # ADR-0058 — derivation starts from the north-dropped remainder: a north
+    # plane never contributes panels to a Sub-Ladder rung, even when its
+    # per-panel yield would win the fill-by-generation ordering.
+
+    # Arrange — smallest rung = a due-north panel (900/panel, best yield) plus
+    # a south plane (4 panels, 1278); max 5 → cap 3.5; after the north drop
+    # the 4-panel south remainder still exceeds the cap → Sub-Ladder fires.
+    north = SolarRoofSegment(
+        segment_index=0,
+        panels_count=1,
+        azimuth_degrees=2.0,
+        pitch_degrees=30.0,
+        yearly_energy_dc_kwh=900.0,
+    )
+    south = SolarRoofSegment(
+        segment_index=1,
+        panels_count=4,
+        azimuth_degrees=180.0,
+        pitch_degrees=30.0,
+        yearly_energy_dc_kwh=1278.0,
+    )
+    potential = SolarPotential(
+        panel_capacity_watts=400.0,
+        max_array_panels_count=5,
+        configurations=(
+            SolarPanelConfiguration(
+                panels_count=5,
+                yearly_energy_dc_kwh=2178.0,
+                segments=(north, south),
+            ),
+        ),
+    )
+
+    # Act
+    configs = select_conservative_configs(potential)
+
+    # Assert — rungs 2 and 3 built from the south plane alone, pro-rata off
+    # its 1278 (639 / 958.5); the 900-yield north panel never appears.
+    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 all(s.azimuth_degrees == 180.0 for c in configs for s in c.segments)