refactor(modelling): expose cascade_scores for the role-3 + bill cascade

Pull the cumulative-prefix scoring out of `marginal_impacts` into a reusable
`cascade_scores(scorer, baseline, overlays) -> list[Score]` (index 0 the
baseline, one calculator run per prefix) plus a pure `marginals_from_scores`.
Each Score carries its SapResult, so the next slice's telescoping per-measure
bill cascade can re-bill the same prefixes the role-3 attribution already
scores — no extra `calculate` calls (ADR-0014 / ADR-0016). `marginal_impacts`
now delegates; behaviour unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

domain/modelling/scoring/scoring.py

@@ -34,17 +34,32 @@ class MeasureImpact:
     energy_savings_kwh_per_yr: float
 
 
-def marginal_impacts(
+def cascade_scores(
     scorer: PackageScorer,
     baseline: EpcPropertyData,
     overlays: Sequence[EpcSimulation],
-) -> list[MeasureImpact]:
-    """Apply overlays cumulatively in order; return each one's marginal impact
-    over the running state. The marginals telescope to the whole-package total."""
+) -> list[Score]:
+    """Score the cumulative prefixes of `overlays` in order: index 0 is the
+    baseline (empty prefix), index i the state after the first i overlays. The
+    list has `len(overlays) + 1` entries — one calculator run each.
+
+    Each Score carries its `SapResult`, so the same cascade powers both the
+    role-3 marginal attribution (`marginals_from_scores`) and the telescoping
+    per-measure bill cascade — neither needs to re-score (ADR-0014 / ADR-0016)."""
+    return [
+        scorer.score(baseline, list(overlays[:prefix_length]))
+        for prefix_length in range(len(overlays) + 1)
+    ]
+
+
+def marginals_from_scores(scores: Sequence[Score]) -> list[MeasureImpact]:
+    """Each measure's marginal impact from a precomputed cumulative-prefix
+    cascade (`scores[0]` is the baseline). Signed so positive is an improvement;
+    the marginals telescope to `scores[-1]` vs `scores[0]`."""
     impacts: list[MeasureImpact] = []
-    previous: Score = scorer.score(baseline, [])
-    for index in range(len(overlays)):
-        current: Score = scorer.score(baseline, list(overlays[: index + 1]))
+    for index in range(1, len(scores)):
+        previous: Score = scores[index - 1]
+        current: Score = scores[index]
         impacts.append(
             MeasureImpact(
                 sap_points=current.sap_continuous - previous.sap_continuous,
@@ -55,10 +70,19 @@ def marginal_impacts(
                 ),
             )
         )
-        previous = current
     return impacts
 
 
+def marginal_impacts(
+    scorer: PackageScorer,
+    baseline: EpcPropertyData,
+    overlays: Sequence[EpcSimulation],
+) -> list[MeasureImpact]:
+    """Apply overlays cumulatively in order; return each one's marginal impact
+    over the running state. The marginals telescope to the whole-package total."""
+    return marginals_from_scores(cascade_scores(scorer, baseline, overlays))
+
+
 def independent_option_impacts(
     scorer: PackageScorer,
     baseline: EpcPropertyData,

tests/domain/modelling/test_scoring.py

@@ -14,8 +14,10 @@ from domain.modelling.scoring.package_scorer import PackageScorer, Score
 from domain.modelling.recommendation import MeasureOption
 from domain.modelling.scoring.scoring import (
     MeasureImpact,
+    cascade_scores,
     independent_option_impacts,
     marginal_impacts,
+    marginals_from_scores,
 )
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
 from domain.sap10_calculator.calculator import Sap10Calculator
@@ -113,6 +115,46 @@ def test_single_overlay_marginal_is_its_improvement_over_baseline() -> None:
     )
 
 
+def test_cascade_scores_returns_the_baseline_plus_one_score_per_prefix() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()
+    scorer = _CountingScorer()
+    overlays = [_MAIN_CAVITY, _EXT1_CAVITY]
+
+    # Act
+    scores: list[Score] = cascade_scores(scorer, baseline, overlays)
+
+    # Assert
+    # baseline (empty prefix) + one score per cumulative prefix
+    assert len(scores) == 3
+    assert scorer.calls == 3
+    assert scores[0].sap_continuous == 0.0  # empty prefix
+    assert scores[1].sap_continuous == 2.0  # MAIN cavity (type 2)
+    assert scores[2].sap_continuous == 4.0  # + EXTENSION_1 cavity (type 2)
+
+
+def test_marginals_from_scores_are_the_consecutive_prefix_deltas() -> None:
+    # Arrange
+    baseline: EpcPropertyData = build_epc()
+    scorer = PackageScorer(Sap10Calculator())
+    overlays = [_MAIN_CAVITY, _EXT1_CAVITY]
+    scores: list[Score] = cascade_scores(scorer, baseline, overlays)
+
+    # Act
+    impacts: list[MeasureImpact] = marginals_from_scores(scores)
+
+    # Assert — each marginal is the delta over the previous prefix score
+    assert len(impacts) == 2
+    assert (
+        abs(impacts[0].sap_points - (scores[1].sap_continuous - scores[0].sap_continuous))
+        <= 1e-9
+    )
+    assert (
+        abs(impacts[1].sap_points - (scores[2].sap_continuous - scores[1].sap_continuous))
+        <= 1e-9
+    )
+
+
 def test_marginals_telescope_to_the_whole_package_total() -> None:
     # Arrange
     baseline: EpcPropertyData = build_epc()