Slice 21e: §4 water heating cascade pins (42/54 PASS)

Extracts `water_heating_section_from_cert(epc) -> WaterHeatingResult`
helper to expose the full §4 cascade output for tests (mirrors the
existing private `_water_heating_worksheet_and_gains`, drops unused
args).

§4 pins at abs=1e-4:
  scalar (2 line refs × 6 = 12): (42) occupancy, (43) annual avg L/day
  monthly (7 line refs × 6 × 12 months = 504 assertions across 42
   parametrized cases): (44)m daily, (45)m energy content,
   (46)m distribution loss, (61)m combi loss, (62)m total demand,
   (64)m output, (65)m heat gains

Per-fixture results:
  000474:    9/9 PASS  ✓
  000477:    5/9       — combi loss (61)m diverges → cascades to
                          62/64/65 monthly
  000480:    9/9 PASS  ✓
  000487:    1/9       — LINE_43 + every monthly fails (HW lodgement
                          defect: number_baths=1 but PDF arithmetic
                          suggests different shower/bath profile)
  000490:    9/9 PASS  ✓
  000516:    9/9 PASS  ✓

4/6 fixtures close §4 fully — strong cascade floor. The 000477 combi
loss residual is a specific Table 3c sub-row issue; the 000487 §4 gap
is part of its broader cert lodgement defect (RR + HW lodgement).

Cumulative scoreboard:
  §1: 12 PASS / 0 FAIL
  §2: 96 PASS / 0 FAIL
  §3:  1 PASS / 23 FAIL  (precision residuals + 000487 RR)
  §4: 42 PASS / 12 FAIL
  ---
  total: 151 PASS / 35 FAIL

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/domain/src/domain/sap/rdsap/cert_to_inputs.py

@@ -691,6 +691,48 @@ def _ventilation_counts(vent: Optional[SapVentilation]) -> _VentilationCounts:
     )
 
 
+def water_heating_section_from_cert(
+    epc: EpcPropertyData,
+) -> Optional[WaterHeatingResult]:
+    """SAP 10.2 §4 cert→inputs cascade. Returns the final
+    `WaterHeatingResult` (every (42)..(65) line ref breakdown) after
+    PCDB Table 3b/3c combi-loss override, exactly as cert_to_inputs
+    computes internally.
+
+    Returns `None` when TFA is missing — the legacy fallback path
+    bypasses §4 entirely; tests using this helper should skip those
+    fixtures.
+    """
+    if epc.total_floor_area_m2 is None:
+        return None
+    main = _first_main_heating(epc)
+    pcdb_main = (
+        gas_oil_boiler_record(main.main_heating_index_number)
+        if main is not None and main.main_heating_index_number is not None
+        else None
+    )
+    bootstrap = water_heating_from_cert(
+        epc=epc,
+        mixer_shower_flow_rates_l_per_min=_mixer_shower_flow_rates_from_cert(epc),
+        has_bath=_has_bath_from_cert(epc),
+        cold_water_temps_c=TABLE_J1_TCOLD_FROM_MAINS_C,
+        low_water_use=False,
+    )
+    combi_loss_override = pcdb_combi_loss_override(
+        pcdb_main,
+        energy_content_monthly_kwh=bootstrap.energy_content_monthly_kwh,
+        daily_hot_water_monthly_l_per_day=bootstrap.daily_hot_water_l_per_day_monthly,
+    )
+    return water_heating_from_cert(
+        epc=epc,
+        mixer_shower_flow_rates_l_per_min=_mixer_shower_flow_rates_from_cert(epc),
+        has_bath=_has_bath_from_cert(epc),
+        cold_water_temps_c=TABLE_J1_TCOLD_FROM_MAINS_C,
+        low_water_use=False,
+        combi_loss_monthly_kwh_override=combi_loss_override,
+    )
+
+
 def heat_transmission_section_from_cert(epc: EpcPropertyData) -> HeatTransmission:
     """SAP 10.2 §3 cert→inputs cascade for `heat_transmission_from_cert`.
 

packages/domain/src/domain/sap/worksheet/tests/test_section_cascade_pins.py

@@ -20,6 +20,7 @@ from domain.sap.rdsap.cert_to_inputs import (
     cert_to_inputs,
     heat_transmission_section_from_cert,
     ventilation_from_cert,
+    water_heating_section_from_cert,
 )
 from domain.sap.worksheet.dimensions import dimensions_from_cert
 from domain.sap.worksheet.tests import (
@@ -233,3 +234,79 @@ def test_section_3_line_refs_match_pdf(
 
     # Assert
     _pin(actual, expected, f"§3 {fixture_attr} {fixture_name}")
+
+
+# ============================================================================
+# §4 Water heating — LINE_42..LINE_65 scalar + monthly tuples
+# ============================================================================
+
+_SECTION_4_SCALAR_PINS: Final[tuple[tuple[str, str], ...]] = (
+    ("LINE_42_OCCUPANCY",                       "occupancy"),
+    ("LINE_43_ANNUAL_AVG_HW_USAGE_L_PER_DAY",   "annual_avg_hot_water_l_per_day"),
+)
+
+_SECTION_4_MONTHLY_PINS: Final[tuple[tuple[str, str], ...]] = (
+    ("LINE_44_M_DAILY_HW_USAGE_L",          "daily_hot_water_l_per_day_monthly"),
+    ("LINE_45_M_HW_ENERGY_CONTENT_KWH",     "energy_content_monthly_kwh"),
+    ("LINE_46_M_DISTRIBUTION_LOSS_KWH",     "distribution_loss_monthly_kwh"),
+    ("LINE_61_M_COMBI_LOSS_KWH",            "combi_loss_monthly_kwh"),
+    ("LINE_62_M_TOTAL_WH_KWH",              "total_demand_monthly_kwh"),
+    ("LINE_64_M_OUTPUT_FROM_WH_KWH",        "output_monthly_kwh"),
+    ("LINE_65_M_HEAT_GAINS_FROM_WH_KWH",    "heat_gains_monthly_kwh"),
+)
+
+
+@pytest.mark.parametrize(
+    "fixture_name,fixture_attr,result_attr",
+    [
+        (fix, line, attr)
+        for fix in _FIXTURES
+        for line, attr in _SECTION_4_SCALAR_PINS
+    ],
+    ids=lambda x: x if isinstance(x, str) else None,
+)
+def test_section_4_scalar_line_refs_match_pdf(
+    fixture_name: str, fixture_attr: str, result_attr: str
+) -> None:
+    """§4 scalar pins — (42) occupancy + (43) annual avg HW usage."""
+    # Arrange
+    mod = _FIXTURES[fixture_name]
+    epc = mod.build_epc()  # type: ignore[attr-defined]
+    expected = getattr(mod, fixture_attr)
+
+    # Act
+    wh = water_heating_section_from_cert(epc)
+    assert wh is not None, f"{fixture_name}: water_heating_from_cert returned None"
+    actual = getattr(wh, result_attr)
+
+    # Assert
+    _pin(actual, expected, f"§4 {fixture_attr} {fixture_name}")
+
+
+@pytest.mark.parametrize(
+    "fixture_name,fixture_attr,result_attr",
+    [
+        (fix, line, attr)
+        for fix in _FIXTURES
+        for line, attr in _SECTION_4_MONTHLY_PINS
+    ],
+    ids=lambda x: x if isinstance(x, str) else None,
+)
+def test_section_4_monthly_line_refs_match_pdf(
+    fixture_name: str, fixture_attr: str, result_attr: str
+) -> None:
+    """§4 monthly pins — daily HW use, energy content, distribution
+    loss, combi loss, total demand, output, heat gains."""
+    # Arrange
+    mod = _FIXTURES[fixture_name]
+    epc = mod.build_epc()  # type: ignore[attr-defined]
+    expected = getattr(mod, fixture_attr)
+
+    # Act
+    wh = water_heating_section_from_cert(epc)
+    assert wh is not None, f"{fixture_name}: water_heating_from_cert returned None"
+    actual = getattr(wh, result_attr)
+
+    # Assert
+    for m in range(12):
+        _pin(actual[m], expected[m], f"§4 {fixture_attr}[{m+1}] {fixture_name}")