test(pcdb): PSR-extension toward 100% above largest / below smallest 🟥

SAP 10.2 Appendix N2 (PDF p.101, footnote 44/45): for an air/ground/water
source heat pump whose plant size ratio exceeds the record's largest PSR,
the efficiency is reciprocal-interpolated between the largest-PSR value and
100% at twice the largest PSR (100% beyond that); below the smallest PSR the
efficiency is 100%. Our interpolator instead clamps to the top/bottom row.

Anchored to the accredited Elmhurst worksheet for cert 100110101713 (golden
fixture case 56, PCDB 100061): PSR 3.10665 over the record's largest 2.0
gives eta_space,1 = 147.011 -> (206) = 0.95 x 147.011 = 139.660, vs the
clamped 352.0 -> 334.4% that over-rates the dwelling by +18 SAP.

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

tests/domain/sap10_calculator/test_pcdb_table_362_lookup.py

@@ -180,3 +180,88 @@ def test_interpolate_heat_pump_efficiency_at_cert_0380_psr_per_sap_app_n() -> No
     #                 ≈ 0.0035077 → eta_water_3 ≈ 285.0861
     assert abs(eta_space_1 - 234.5235) < 1e-3
     assert abs(eta_water_3 - 285.0861) < 1e-3
+
+
+def test_interpolate_extends_above_largest_psr_toward_100pct_per_app_n() -> None:
+    """SAP 10.2 Appendix N2 (PDF p.101, footnote 44/45) — PSR above the
+    record's largest value extends the efficiency toward 100%, it is NOT
+    clamped to the top-of-table value.
+
+        "in the case of a heat pump (ground, water or air source), where
+        the PSR is greater than the largest value in the data record, an
+        efficiency may be obtained from linear interpolation between that
+        at the largest PSR in the data record and efficiency 100% at PSR
+        two times the largest PSR in the data record. If the PSR is
+        greater than two times the largest PSR in the data record an
+        efficiency of 100% should be used."
+
+    Interpolation is reciprocal-linear (footnote 43). Accredited anchor:
+    Elmhurst worksheet for cert 100110101713 / "golden fixture debugging"
+    case 56 (PCDB 100061, ECODAN 8.5 kW, largest PSR row η_space,1=352.0).
+    The dwelling HLC (39) = 107.8199 W/K and max output 8.106 kW give
+
+        PSR = 8.106 × 1000 / (107.8199 × 24.2) = 3.106650
+
+    which exceeds the record's largest PSR (2.0). The spec extension to
+    100% at 2 × 2.0 = 4.0 yields, at t = (3.106650 − 2.0)/(4.0 − 2.0):
+
+        1/η = (1 − t)/352.0 + t/100.0  →  η_space,1 = 147.011
+
+    so that (206) = 0.95 × 147.011 = 139.660 — matching the accredited
+    worksheet exactly. The previous top-of-table clamp returned 352.0
+    (→ 334.4%), over-rating the dwelling by +18 SAP.
+    """
+    from domain.sap10_calculator.tables.pcdb.parser import (
+        interpolate_heat_pump_efficiency_at_psr,
+    )
+
+    record = heat_pump_record(100061)
+    assert record is not None
+    assert record.psr_groups[-1].psr == 2.0
+    assert record.psr_groups[-1].eta_space_1_pct == 352.0
+
+    eta_space_1, _eta_water_3 = interpolate_heat_pump_efficiency_at_psr(
+        record.psr_groups, target_psr=3.106649864134083,
+    )
+
+    assert abs(eta_space_1 - 147.011) < 1e-2
+    assert abs(0.95 * eta_space_1 - 139.6604) < 1e-2
+
+
+def test_interpolate_above_twice_largest_psr_is_100pct_per_app_n() -> None:
+    """SAP 10.2 Appendix N2 — beyond twice the largest PSR the efficiency
+    is exactly 100% (the upper terminus of the extension), for both space
+    and water heating PSR-dependent results."""
+    from domain.sap10_calculator.tables.pcdb.parser import (
+        interpolate_heat_pump_efficiency_at_psr,
+    )
+
+    record = heat_pump_record(100061)
+    assert record is not None
+
+    eta_space_1, eta_water_3 = interpolate_heat_pump_efficiency_at_psr(
+        record.psr_groups, target_psr=9.0,  # > 2 × 2.0
+    )
+
+    assert eta_space_1 == 100.0
+    assert eta_water_3 == 100.0
+
+
+def test_interpolate_below_smallest_psr_is_100pct_per_app_n() -> None:
+    """SAP 10.2 Appendix N2 (PDF p.101) — "For all heat pumps, an
+    efficiency of 100% should be used if the PSR is less than the smallest
+    value in the database record." (Not clamped to the smallest row.)"""
+    from domain.sap10_calculator.tables.pcdb.parser import (
+        interpolate_heat_pump_efficiency_at_psr,
+    )
+
+    record = heat_pump_record(100061)
+    assert record is not None
+    assert record.psr_groups[0].psr == 0.2
+
+    eta_space_1, eta_water_3 = interpolate_heat_pump_efficiency_at_psr(
+        record.psr_groups, target_psr=0.1,  # < 0.2
+    )
+
+    assert eta_space_1 == 100.0
+    assert eta_water_3 == 100.0