feat(overrides): GREEN — BoilerEfficiencyBand enum + SEDBUK band parser

BoilerEfficiencyBand (A-G + UNKNOWN sentinel) and boiler_efficiency_band_guard,
the shared deterministic parser for the fifth Heating Companion (ADR-0068).
UNKNOWN is never persisted; absence of a band -> no value -> Table 4b default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

domain/epc/property_overrides/boiler_efficiency_band.py

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+from enum import Enum
+
+
+class BoilerEfficiencyBand(Enum):
+    """The SEDBUK A-G efficiency band a landlord carries in the Boiler Landlord
+    Description (``Boiler: C rated Combi``) — the fifth Heating Companion
+    (ADR-0068).
+
+    An *attribute* of a ``main_heating_system`` override, orthogonal to the
+    ``MainHeatingSystemType`` archetype: the archetype names the *system*, the
+    band names *how well it burns*. It is deliberately NOT a per-band archetype
+    — that enum is FE-owned ([[main-heating-system-pgenum-is-fe-owned]]), and 7
+    bands x N boiler types would explode a cross-team contract.
+
+    Each band resolves (via the main-heating overlay) to a ``(winter, summer)``
+    seasonal-efficiency pair the calculator consumes ahead of the Table 4b code
+    default — applied to the SEDBUK-rated wet boilers only (gas/LPG/biogas and
+    oil); solid-fuel / electric / CPSU boilers carry an intrinsic fixed
+    efficiency and take no band.
+
+    ``UNKNOWN`` is the classifier's "no band recognised" sentinel (a plain
+    boiler, or a non-boiler heating system). It is never persisted as an
+    override — absence of a band is modelled as *no value*, so the boiler keeps
+    its Table 4b code default — mirroring how the finaliser drops the other
+    ``UNKNOWN`` classifier outputs.
+    """
+
+    A = "A"
+    B = "B"
+    C = "C"
+    D = "D"
+    E = "E"
+    F = "F"
+    G = "G"
+    UNKNOWN = "Unknown"

domain/epc/property_overrides/boiler_efficiency_band_guard.py

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+"""Parse the SEDBUK A-G efficiency band out of a Landlord "Heating" description.
+
+The single source of truth for the band, shared by three call sites so they
+cannot drift (mirroring how ``main_heating_guard`` backs both the live
+classifier path and ``reclassify_main_heating``):
+
+* the modelling read path (``overlays_from`` → the main-heating overlay slot),
+* the ``boiler_efficiency_band`` backfill,
+* the live classifier column (as its deterministic guard).
+
+The Landlord "Heating" column carries the band as ``Boiler: <A-G> rated
+<Combi|Regular Boiler|NA>``. A cell can name a second system after ``System 2:``;
+the primary (system 1) band leads the cell, so the first ``Boiler: <letter>
+rated`` occurrence is the one that scores the dwelling's main system. A
+description with no band phrasing (a plain boiler, or a non-boiler heating
+system) returns ``None`` — absence of a band, not a guess.
+"""
+
+from __future__ import annotations
+
+import re
+from typing import Optional
+
+from domain.epc.property_overrides.boiler_efficiency_band import BoilerEfficiencyBand
+
+# `Boiler: <A-G> rated` — the SEDBUK band phrasing. Case-insensitive (the
+# backfill lowercases the stored description). `\b` after the letter rejects a
+# stray "H rated" or a multi-letter token; the leading "Boiler:" anchors it so a
+# bare "A rated" elsewhere in free text cannot match.
+_BAND_RE = re.compile(r"boiler:\s*([a-g])\s+rated\b", re.IGNORECASE)
+
+
+def boiler_efficiency_band_guard(description: str) -> Optional[BoilerEfficiencyBand]:
+    """The SEDBUK band of the primary (system 1) boiler in a Landlord "Heating"
+    description, or ``None`` when the description carries no band phrasing."""
+    match = _BAND_RE.search(description)
+    if match is None:
+        return None
+    return BoilerEfficiencyBand(match.group(1).upper())