A dwelling with no fabric candidates proceeds straight to the full pool 🟩

Documents behaviour already delivered: an empty phase 1 meets no target, so
phase 2 optimises every group — identical to a plain run. Test passed on
arrival.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/domain/modelling/test_optimiser_fabric_first.py

@@ -275,6 +275,29 @@ def test_ventilation_dependency_is_injected_once_across_both_phases() -> None:
     assert abs(package.score.sap_continuous - 68.0) <= 1e-9
 
 
+def test_no_fabric_candidates_proceeds_straight_to_the_full_pool() -> None:
+    # Arrange — the envelope work is already done (no fabric Recommendation
+    # survives generation); fabric first must not veto the run, it just means
+    # phase 1 has nothing to do.
+    groups: list[list[ScoredOption]] = [
+        [_scored("air_source_heat_pump", gain=20.0, cost=8000.0, overlay=_HEATING_OVERLAY)],
+    ]
+    scorer = _StubScorer(base=60.0, wall=5.0, roof=0.0, heating=20.0)
+
+    # Act — target 75.
+    package: OptimisedPackage = optimise_package_fabric_first(
+        groups=groups,
+        scorer=scorer,
+        baseline_epc=build_epc(),
+        budget=20000.0,
+        target_sap=75.0,
+    )
+
+    # Assert — the heat pump package, exactly as a plain run would produce.
+    assert _selected_types(package) == {"air_source_heat_pump"}
+    assert abs(package.score.sap_continuous - 80.0) <= 1e-9
+
+
 class _InteractionScorer:
     """A stub whose boiler gain collapses once the wall is insulated (+10 raw,
     +3 post-fabric) while the heat pump's holds (+8 either way) — so a phase 2