feat(modelling): Epc.sap_lower_bound() — band → minimum SAP (#1160)

Slice 3a. The inverse of Epc.from_sap_score: the minimum SAP rating in a
band (C → 69, B → 81, …), used as the Optimiser's repair target for an
INCREASING_EPC goal (goal_value "C" → target SAP 69). Keeps the
band-target derivation in the domain rather than re-coupling to
backend.app.utils.epc_to_sap_lower_bound. 8 tests incl. round-trip
through from_sap_score; pyright strict clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

datatypes/epc/domain/epc.py

@@ -31,3 +31,18 @@ class Epc(Enum):
         if score >= 21:
             return cls.F
         return cls.G
+
+    def sap_lower_bound(self) -> int:
+        """The minimum SAP rating in this band — the inverse of
+        `from_sap_score` (A → 92, B → 81, C → 69, D → 55, E → 39, F → 21,
+        G → 1). Used as an optimisation target, e.g. "reach band C" → 69."""
+        bounds: dict["Epc", int] = {
+            Epc.A: 92,
+            Epc.B: 81,
+            Epc.C: 69,
+            Epc.D: 55,
+            Epc.E: 39,
+            Epc.F: 21,
+            Epc.G: 1,
+        }
+        return bounds[self]

tests/datatypes/epc/domain/test_epc.py

@@ -0,0 +1,26 @@
+"""Behaviour of the Epc band enum's SAP mapping — the band a SAP rating falls
+in, and the minimum SAP rating of a band (the inverse, used as an optimisation
+target)."""
+
+from __future__ import annotations
+
+import pytest
+
+from datatypes.epc.domain.epc import Epc
+
+
+def test_sap_lower_bound_returns_the_band_floor() -> None:
+    # Act / Assert — the standard SAP10 band floors.
+    assert Epc.A.sap_lower_bound() == 92
+    assert Epc.B.sap_lower_bound() == 81
+    assert Epc.C.sap_lower_bound() == 69
+    assert Epc.D.sap_lower_bound() == 55
+    assert Epc.E.sap_lower_bound() == 39
+    assert Epc.F.sap_lower_bound() == 21
+    assert Epc.G.sap_lower_bound() == 1
+
+
+@pytest.mark.parametrize("band", list(Epc))
+def test_band_floor_round_trips_through_from_sap_score(band: Epc) -> None:
+    # Act / Assert — a band's floor scores back to that band.
+    assert Epc.from_sap_score(band.sap_lower_bound()) is band