Flag works bought for no real SAP gain 🟩

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

scripts/audit/anomalies.py

@@ -26,6 +26,11 @@ be justified against the real distribution, not guessed.
 Read-only: this script never writes to the DB.
 """
 
+# Every check is reached through `_REGISTRY` via the `@check` decorator, never by
+# name, so pyright sees each one as an unused function. That is the design, not a
+# defect — suppress it for the module rather than tagging a dozen definitions.
+# pyright: reportUnusedFunction=false
+
 from __future__ import annotations
 
 import argparse
@@ -42,6 +47,11 @@ from scripts.e2e_common import build_engine, load_env
 # A..G, A best — index is the rank (lower = better) for band comparisons.
 _BANDS = "ABCDEFG"
 
+# The smallest SAP movement that counts as a real improvement. The effective
+# baseline is a rounded integer, so anything at or under half a point is within
+# the rounding the published band already absorbs — invisible on a certificate.
+_MIN_REAL_SAP_GAIN = 0.5
+
 
 def _band_rank(band: Optional[str]) -> Optional[int]:
     if band is None or band not in _BANDS:
@@ -189,7 +199,34 @@ def _already_meets_goal_with_works(a: PropertyAudit) -> Optional[str]:
 
 @check("costed-plan-without-real-gain", Severity.HIGH)
 def _costed_plan_without_real_gain(a: PropertyAudit) -> Optional[str]:
-    raise NotImplementedError
+    """Money was spent but the modelled post-retrofit SAP barely moves off the
+    effective baseline — works that buy no real improvement.
+
+    Provenance: #1652 / #1655. This closes a coverage hole rather than adding a
+    new opinion: `plan-score-below-baseline` only fires on a *drop* beyond 0.5,
+    and `zero-works-post-differs` only on £0 plans, so a **costed** plan landing
+    flat against the baseline was caught by neither. It is what the 112
+    "no real improvement" homes in portfolio 796 looked like — priced plans whose
+    post-retrofit SAP sat ~0.2 *below* the effective baseline — and what the
+    band-boundary defect (#1654) produced at scale: 630 homes each sold ~1 SAP
+    point of works toward a band they had already reached.
+
+    The 0.5 threshold is the same "within rounding is not a real move" line the
+    published banding uses: the effective baseline is a rounded integer, so a
+    sub-half-point shift is not a change anyone can see on a certificate. Spend
+    that buys less than that is not a retrofit."""
+    if a.effective_sap is None or a.post_sap is None:
+        return None
+    cost = a.cost_of_works or 0.0
+    if cost <= 0.0:
+        return None
+    gain = a.post_sap - a.effective_sap
+    if gain > _MIN_REAL_SAP_GAIN:
+        return None
+    return (
+        f"£{cost:.0f} of works for {gain:+.1f} SAP "
+        f"(effective {a.effective_sap:.1f} → post {a.post_sap:.1f})"
+    )
 
 
 @check("plan-stops-short-of-goal", Severity.HIGH)
@@ -555,11 +592,14 @@ def _load(
             rollups = {
                 m["property_id"]: (m["solar_sap"], m["solar_bill"], m["n_measures"])
                 for m in (
-                    row._mapping for row in conn.execute(_ROLLUP_QUERY, params)
+                    # `Row._mapping` is public API in SQLAlchemy 2.0; only the
+                    # stub marks it protected.
+                    row._mapping  # pyright: ignore[reportPrivateUsage]
+                    for row in conn.execute(_ROLLUP_QUERY, params)
                 )
             }
         for r in conn.execute(_QUERY, params):
-            m = r._mapping
+            m = r._mapping  # pyright: ignore[reportPrivateUsage]
             solar_sap, solar_bill, n_measures = rollups.get(m["id"], (None, None, 0))
             out.append(
                 PropertyAudit(