Slice 26: §7 LINE_92/93 closure — RdSAP §15 area rounding on living area

LINE_91 in the worksheet is `living_area / (4)`, where living_area itself
is the §15-rounded materialisation of `Table 27 fraction × TFA`. RdSAP
§9.2 (p.52): "The living area is then the fraction multiplied by the
total floor area." §15 (p.66) lists "All internal floor areas and living
area: 2 d.p." So the actual LINE_91 fed to the §7 zone blend is
`round_half_up(Table_27 × TFA, 2) / TFA`, not the raw Table 27 entry.

The roundtrip explains why the 4 holdout fixtures lodge LINE_91 = 0.3001
or 0.2501 instead of the Table 27 values 0.30 / 0.25:
  000474: 0.30 × 56.79 → 17.04 / 56.79 = 0.3001
  000477: 0.25 × 77.58 → 19.40 / 77.58 = 0.2501
  000490: 0.25 × 66.06 → 16.52 / 66.06 = 0.2501

`_living_area_fraction` now takes TFA and materialises + rounds + divides;
`_living_area_fraction_default` retains the bare Table 27 lookup. Existing
`_round_half_up` from heat_transmission is the right utility (same §15
boundary, same half-up convention).

Scoreboard: §7 cascade pins 52/60 → 60/60 (closes LINE_92/93 on 000474,
000477, 000480, 000490 — and tightens the already-passing 000487/000516
combinations). Full cascade: 304/312 → 312/312 (100%).

e2e SapResult: 27/66 → 56/66 (continuous SAP, ECF, fuel cost, space
heating kWh now close on 5/6 fixtures; 000487 still has unrelated
downstream defects, all 6 CO2 fails await §12).

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

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

@@ -84,6 +84,8 @@ from domain.sap.worksheet.solar_gains import (
 from domain.sap.worksheet.heat_transmission import (
     DwellingExposure,
     HeatTransmission,
+    _AREA_ROUND_DP,
+    _round_half_up,
     heat_transmission_from_cert,
 )
 from domain.sap.climate.appendix_u import external_temperature_c
@@ -348,9 +350,9 @@ def _is_timber_or_steel_frame(parts: list[SapBuildingPart]) -> bool:
     return isinstance(wc, int) and wc in (5, 6)
 
 
-def _living_area_fraction(habitable_rooms_count: Optional[int]) -> float:
-    """RdSAP 10 Table 27 by `habitable_rooms_count`. Defaults to the
-    bottom of the table for ≥8 rooms; falls back to the SAP convention
+def _living_area_fraction_default(habitable_rooms_count: Optional[int]) -> float:
+    """RdSAP 10 Table 27 (p.52) lookup by `habitable_rooms_count`. Defaults
+    to the bottom of the table for ≥8 rooms; falls back to the SAP convention
     0.21 when count missing or zero."""
     if not habitable_rooms_count or habitable_rooms_count <= 0:
         return _LIVING_AREA_FRACTION_DEFAULT
@@ -359,6 +361,25 @@ def _living_area_fraction(habitable_rooms_count: Optional[int]) -> float:
     return _LIVING_AREA_FRACTION_MIN
 
 
+def _living_area_fraction(
+    habitable_rooms_count: Optional[int], total_floor_area_m2: float
+) -> float:
+    """SAP 10.2 §7 LINE_91 = Living area / TFA.
+
+    RdSAP §9.2 (p.52): living area = Table 27 fraction × TFA. RdSAP §15
+    (p.66) requires "All internal floor areas and living area: 2 d.p." at
+    the RdSAP→SAP boundary. So the materialised living area is rounded to
+    2 d.p. half-up, then divided back by TFA to yield the LINE_91 that
+    feeds the §7 zone blend. This roundtrip is why fixtures lodge
+    e.g. 0.3001 (= 17.04/56.79) rather than the raw 0.30 Table 27 entry.
+    """
+    fraction = _living_area_fraction_default(habitable_rooms_count)
+    if total_floor_area_m2 <= 0.0:
+        return fraction
+    living_area_m2 = _round_half_up(fraction * total_floor_area_m2, _AREA_ROUND_DP)
+    return living_area_m2 / total_floor_area_m2
+
+
 def _window_total_area_and_avg_u(windows: list[SapWindow]) -> tuple[float, Optional[float]]:
     """Area-weighted total + U-value for the conduction worksheet."""
     if not windows:
@@ -877,7 +898,9 @@ def mean_internal_temperature_section_from_cert(
         total_floor_area_m2=dim.total_floor_area_m2,
         control_type=_control_type(main),
         responsiveness=_responsiveness(main),
-        living_area_fraction=_living_area_fraction(epc.habitable_rooms_count),
+        living_area_fraction=_living_area_fraction(
+            epc.habitable_rooms_count, dim.total_floor_area_m2
+        ),
         control_temperature_adjustment_c=0.0,
     )
 
@@ -1407,7 +1430,9 @@ def cert_to_inputs(
     # for the Elmhurst corpus (cert-side mapping is a future slice).
     control_type_value = _control_type(main)
     responsiveness_value = _responsiveness(main)
-    living_area_fraction_value = _living_area_fraction(epc.habitable_rooms_count)
+    living_area_fraction_value = _living_area_fraction(
+        epc.habitable_rooms_count, dim.total_floor_area_m2
+    )
     monthly_total_gains_w = tuple(
         internal_gains_monthly_w[m] + solar_gains_monthly_w[m] for m in range(12)
     )