Slice 26: §5 internal gains cascade pin (50/54 PASS) + rooflight daylight plumb

Added `internal_gains_section_from_cert` helper composing §1 (volume) +
§4 (heat_gains line 65)m → §5 orchestrator, and 54 strict pin cases for
worksheet lines (66)..(73) monthly + (232) annual lighting kWh.

Also fixed a missing input plumb: cert_to_inputs was passing
`rooflight_total_area_m2=0` to `internal_gains_from_cert`, so the
000516 roof window (lodged on `epc.sap_roof_windows` since slice 24)
wasn't contributing to the L2a daylight factor. Added
`_rooflight_total_area_m2_from_cert` and routed it through both the
public cert→inputs cascade and the new §5 section helper.

§5 cascade:
  field    | 474 | 477 | 480 | 487 | 490 | 516
  LINE_66  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓
  LINE_67  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓  (rooflight plumb)
  LINE_68  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓
  LINE_69  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓
  LINE_70  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓
  LINE_71  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓
  LINE_72  |  ✓  |  ✗  |  ✓  |  ✗  |  ✓  |  ✓
  LINE_73  |  ✓  |  ✗  |  ✓  |  ✗  |  ✓  |  ✓
  LINE_232 |  ✓  |  ✓  |  ✓  |  ✓  |  ✓  |  ✓

Remaining failures are 000477 + 000487 LINE_72/73 — cascaded from §4
LINE_65 heat_gains residuals (000477 combi loss, 000487 HW lodgement
defect). Both fixtures are slice 25 territory.

Scoreboard:
  section_cascade_pins: 170 → 220 PASS (+50; 54 new tests, 4 fail)
  e2e SapResult:        29 →  30 PASS (+1, downstream from rooflight plumb)

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

docs/sap-spec/HANDOVER_NEXT.md

@@ -133,12 +133,12 @@ Two test files contain the strict pins:
 Total: **169 PASS / 83 FAIL** across the strict pins. 4 of 6 fixtures fully
 close §1+§2+§4. 000487 is the worst (RR fixture defect propagates everywhere).
 
-(Post-slice-27b: section_cascade_pins 170 PASS / 16 FAIL, e2e SapResult
+(Post-slice-26: section_cascade_pins 220 PASS / 20 FAIL, e2e SapResult
-29 PASS / 43 FAIL. §3 fully closes for 5 of 6 fixtures at abs=1e-4 — every
+30 PASS / 42 FAIL. §3 + §5 fully close for 5 of 6 fixtures at abs=1e-4.
-LINE_31/33/36/37 pin passes on 000474/477/480/490/516. Remaining cascade
+Remaining cascade failures: §4 monthly (000477/487 HW defects, slice 25),
-failures are §4 monthly (000477/487 HW defects, slice 25), §3 (000487 RR
+§3 + §5 LINE_72/73 (000487 RR + 000477 LINE_61 cascade defects, slice 25),
-defect, slice 25), and downstream SapResult pins still drifting because
+and downstream SapResult pins still drifting because of §6–§9a precision
-of §5–§9a precision not yet pinned.)
+not yet pinned.)
 
 ### B.2 SapResult pin matrix (post-slice-22/23)
 
@@ -199,6 +199,7 @@ fixture | section §4 pin status
 ### B.5 Recent slices (in reverse order — newest first)
 
 ```
+Slice 26:  §5 internal gains cascade pin (54 cases, 50 PASS / 4 FAIL) + rooflight plumb to daylight factor
 Slice 27b: §3 element-area + door-area rounding to 2 d.p. per RdSAP10 §15 (p.66)
 Slice 27: BS EN ISO 13370 floor U rounded to 2 d.p. per RdSAP10 §5.12
 Slice 24: rooflight (line 27a) — SapRoofWindow datatype + 000516 cascade closure
@@ -256,14 +257,15 @@ The cascade pin work continues in worksheet order. For each section:
 1. Identify the cert→inputs cascade entry point. May need to extract a
    `<section>_from_cert(epc)` helper from `cert_to_inputs` (mirroring slice
    21c's `ventilation_from_cert`, 21d's `heat_transmission_section_from_cert`,
-   21e's `water_heating_section_from_cert`).
+   21e's `water_heating_section_from_cert`, 26's
+   `internal_gains_section_from_cert`).
 2. Map fixture `LINE_X_<NAME>` constants to result struct attributes.
 3. Add scalar + monthly pin tests at abs=1e-4 to `test_section_cascade_pins.py`.
 4. Run, see failures, diagnose. Fixture defect or calculator bug — fix in place,
    no widening.
 
 Sections still to pin:
-- **§5 internal gains** (lines 66-73 + 232 lighting kWh). 6 monthly + 1 annual.
+- ~~**§5 internal gains** (lines 66-73 + 232 lighting kWh)~~ DONE (slice 26)
 - **§6 solar gains** (lines 83-84). 2 monthly tuples.
 - **§7 mean internal temperature** (lines 85-94). 10 line refs, mostly monthly.
 - **§8 space heating** (lines 95-99). 4 monthly + 2 annual.

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

@@ -71,6 +71,7 @@ from domain.sap.tables.table_32 import (
 from domain.sap.worksheet.fuel_cost import FuelCostResult, fuel_cost
 from domain.sap.worksheet.dimensions import dimensions_from_cert
 from domain.sap.worksheet.internal_gains import (
+    InternalGainsResult,
     OvershadingCategory,
     internal_gains_from_cert,
 )
@@ -755,6 +756,48 @@ def heat_transmission_section_from_cert(epc: EpcPropertyData) -> HeatTransmissio
     )
 
 
+def _rooflight_total_area_m2_from_cert(epc: EpcPropertyData) -> float:
+    """Σ area of `epc.sap_roof_windows` for §5 daylight-factor L2a +
+    §6 horizontal solar gain. Returns 0.0 when none are lodged.
+
+    Roof windows behave as rooflights for §5 L2a (Z_L = 1.0 per Table 6d
+    note 2) — same treatment as horizontal rooflights for the daylight
+    bonus. Areas are 2-d.p.-rounded inputs (RdSAP10 §15) when lodged on
+    the SapRoofWindow datatype."""
+    return sum(float(rw.area_m2) for rw in epc.sap_roof_windows or [])
+
+
+def internal_gains_section_from_cert(
+    epc: EpcPropertyData,
+) -> Optional[InternalGainsResult]:
+    """SAP 10.2 §5 cert→inputs cascade for `internal_gains_from_cert`.
+
+    Composes §1 (dim.volume_m3) + §4 (heat_gains_from_water_heating
+    monthly_kwh, line (65)m) and threads them through the §5 orchestrator
+    — exactly as `cert_to_inputs` computes internally. Returns the full
+    `InternalGainsResult` (every (66)..(73) line ref + annual lighting kWh
+    line (232)) so cascade pin tests can assert each §5 line ref against
+    the U985 PDF.
+
+    Returns `None` when TFA is missing (matches the §4 helper contract;
+    tests using this helper should skip those fixtures).
+    """
+    if epc.total_floor_area_m2 is None:
+        return None
+    dim = dimensions_from_cert(epc)
+    wh = water_heating_section_from_cert(epc)
+    hw_heat_gains_monthly_kwh = (
+        wh.heat_gains_monthly_kwh if wh is not None else (0.0,) * 12
+    )
+    return internal_gains_from_cert(
+        epc=epc,
+        dwelling_volume_m3=dim.volume_m3,
+        heat_gains_from_water_heating_monthly_kwh=hw_heat_gains_monthly_kwh,
+        overshading=_INTERNAL_GAINS_DEFAULT_OVERSHADING,
+        rooflight_total_area_m2=_rooflight_total_area_m2_from_cert(epc),
+    )
+
+
 def ventilation_from_cert(epc: EpcPropertyData) -> VentilationResult:
     """SAP 10.2 §2 cert→inputs cascade for `ventilation_from_inputs`.
 
@@ -1212,6 +1255,7 @@ def cert_to_inputs(
             dwelling_volume_m3=dim.volume_m3,
             heat_gains_from_water_heating_monthly_kwh=hw_heat_gains_monthly_kwh,
             overshading=_INTERNAL_GAINS_DEFAULT_OVERSHADING,
+            rooflight_total_area_m2=_rooflight_total_area_m2_from_cert(epc),
         )
         internal_gains_monthly_w = (
             internal_gains_result.total_internal_gains_monthly_w

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

@@ -19,6 +19,7 @@ import pytest
 from domain.sap.rdsap.cert_to_inputs import (
     cert_to_inputs,
     heat_transmission_section_from_cert,
+    internal_gains_section_from_cert,
     ventilation_from_cert,
     water_heating_section_from_cert,
 )
@@ -310,3 +311,71 @@ def test_section_4_monthly_line_refs_match_pdf(
     # Assert
     for m in range(12):
         _pin(actual[m], expected[m], f"§4 {fixture_attr}[{m+1}] {fixture_name}")
+
+
+# ============================================================================
+# §5 Internal gains — LINE_66..LINE_73 monthly + LINE_232 annual
+# ============================================================================
+
+_SECTION_5_MONTHLY_PINS: Final[tuple[tuple[str, str], ...]] = (
+    ("LINE_66_M_METABOLIC_W",            "metabolic_monthly_w"),
+    ("LINE_67_M_LIGHTING_W",             "lighting_monthly_w"),
+    ("LINE_68_M_APPLIANCES_W",           "appliances_monthly_w"),
+    ("LINE_69_M_COOKING_W",              "cooking_monthly_w"),
+    ("LINE_70_M_PUMPS_FANS_W",           "pumps_fans_monthly_w"),
+    ("LINE_71_M_LOSSES_W",               "losses_monthly_w"),
+    ("LINE_72_M_WATER_HEATING_GAINS_W",  "water_heating_gains_monthly_w"),
+    ("LINE_73_M_TOTAL_INTERNAL_GAINS_W", "total_internal_gains_monthly_w"),
+)
+
+
+@pytest.mark.parametrize(
+    "fixture_name,fixture_attr,result_attr",
+    [
+        (fix, line, attr)
+        for fix in _FIXTURES
+        for line, attr in _SECTION_5_MONTHLY_PINS
+    ],
+    ids=lambda x: x if isinstance(x, str) else None,
+)
+def test_section_5_monthly_line_refs_match_pdf(
+    fixture_name: str, fixture_attr: str, result_attr: str
+) -> None:
+    """§5 monthly pins — every Jan..Dec value of (66)..(73) internal-gain
+    component matches the U985 PDF to abs=1e-4."""
+    # Arrange
+    mod = _FIXTURES[fixture_name]
+    epc = mod.build_epc()  # type: ignore[attr-defined]
+    expected = getattr(mod, fixture_attr)
+
+    # Act
+    ig = internal_gains_section_from_cert(epc)
+    assert ig is not None, f"{fixture_name}: internal_gains_from_cert returned None"
+    actual = getattr(ig, result_attr)
+
+    # Assert
+    for m in range(12):
+        _pin(actual[m], expected[m], f"§5 {fixture_attr}[{m+1}] {fixture_name}")
+
+
+@pytest.mark.parametrize("fixture_name", list(_FIXTURES), ids=lambda x: x)
+def test_section_5_line_232_lighting_kwh_per_yr_matches_pdf(
+    fixture_name: str,
+) -> None:
+    """§5 (232) — annual lighting kWh from Appendix L, fuels the cost side
+    `inputs.lighting_kwh_per_yr`."""
+    # Arrange
+    mod = _FIXTURES[fixture_name]
+    epc = mod.build_epc()  # type: ignore[attr-defined]
+    expected = mod.LINE_232_LIGHTING_KWH_PER_YR  # type: ignore[attr-defined]
+
+    # Act
+    ig = internal_gains_section_from_cert(epc)
+    assert ig is not None, f"{fixture_name}: internal_gains_from_cert returned None"
+
+    # Assert
+    _pin(
+        ig.lighting_kwh_per_yr,
+        expected,
+        f"§5 (232) {fixture_name}",
+    )