docs: handover update — +2.5 SAP cluster is heterogeneous, not a shared cascade gap

Probed all three variants (electric 3, oil 1, solid fuel 2) in this
thread. Each has a different driver despite the matching magnitude:
- electric 3: §9 useful-demand calc for ctrl=3 storage heaters
- oil 1: HW efficiency for Table 4b oil boiler (cascade 86% vs ws ~65%)
- solid fuel 2: HW kWh lodged in different line ref (re-probe needed)

Tested combined-R hypothesis (effective_R = (1-frac)·R_main + frac·R_sec
per SAP 10.2 §9b) — the cascade currently DOES NOT pass secondary_fraction
to mean_internal_temperature_monthly, so effective_R = R_main. Monkey-
patching to inject combined R REGRESSES electric 3 (+2.55 → +3.17)
because raising R lowers cascade demand — opposite of needed direction.

Recommends taking the three variants as separate per-variant slices.

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

domain/sap10_calculator/docs/HANDOVER_POST_S0380_140.md

@@ -50,29 +50,49 @@ storage 11/12/13/14, no system, oil 2-6, pcdb 3.
 
 ## Next-slice candidates ranked by leverage
 
-### 1. **+2.5 SAP cluster (electric 3, oil 1, solid fuel 2)** — open
+### 1. **+2.5 SAP cluster (electric 3, oil 1, solid fuel 2)** — open, HETEROGENEOUS
 
-These three variants share `ΔSAP_c ≈ +2.55` with `Δcost ≈ −£60`.
-Different SAP codes (401 storage, oil Table 4b, anthracite 158) but
-remarkably consistent residual magnitude — suggests one shared
-cascade gap.
+These three variants share `ΔSAP_c ≈ +2.55` with `Δcost ≈ −£60`, but
+**probing shows the residuals trace to DIFFERENT causes — not a
+single shared cascade gap**. Slice attempted in this thread, abandoned
+after diagnosis showed each variant needs a different fix:
 
-**Diagnosis pending.** Probe results so far:
-- electric 3 SH+Sec demand: cascade 10046 kWh vs worksheet 11088 kWh
-  (cascade under by 1042 kWh = ~10%)
-- Not Table 11 fraction (cascade rate = worksheet rate = 7.41 p/kWh
-  after S0380.138, so SH/Sec swap is cost-neutral)
-- Possibly Table 9 MIT calc — these are all "low-R" systems (R=0.00
-  for 401, R=0.50 for 158, R=0.40-0.60 for oil Table 4b). Cascade
-  may under-compute MIT for low-R systems → lower demand.
+| variant | SAP code | SH+Sec demand gap | HW kWh gap | Driver |
+|---|---|---:|---:|---|
+| electric 3 | 401 | cascade UNDER by 1005 kWh | exact | §9 MIT for storage heaters |
+| oil 1 | 127 | cascade OVER by 104 kWh (small) | cascade UNDER by 854 kWh | HW efficiency 86% vs worksheet 65% |
+| solid fuel 2 | 158 | cascade OVER by 337 kWh | unclear (different lodging) | TBD |
 
-**Suggested slice plan:**
-1. Probe `space_heating_kwh_per_yr` derivation for code 401 vs
-   worksheet (62)/(63) lines
-2. Check Table 9b MIT formula application — does cascade use R
-   correctly in `alpha = 1 - 0.4 × R` or similar?
-3. If MIT is the root, fix the formula
-4. Re-pin all 3 cluster variants + side effects
+**Combined-R hypothesis tested and rejected.** SAP 10.2 §9b defines
+`effective_R = (1−sec_frac) × R_main + sec_frac × R_sec`, which the
+cascade is NOT applying (`mean_internal_temperature_section_from_cert`
+at L2543 and main orchestrator at L4490 both call
+`mean_internal_temperature_monthly` without passing
+`secondary_fraction`/`secondary_responsiveness`). A monkey-patch to
+inject these REGRESSES electric 3 (+2.55 → +3.17) because raising
+effective_R LOWERS cascade demand — the wrong direction. The cascade
+is "compensating by not using combined R" — fixing this in isolation
+will require fixing the demand undercount simultaneously.
+
+**Per-variant fix plans:**
+- **electric 3**: cascade total useful demand (10046 kWh) is ~10%
+  below worksheet (11088 kWh). Not driven by R alone — even with
+  combined-R fix it gets worse. Likely the cascade's Table 9 heating-
+  hours-per-day for ctrl=3 storage heaters differs from worksheet
+  (worksheet may assume 24-hr "always on" for off-peak storage; cascade
+  uses the standard ctrl=3 pattern).
+- **oil 1**: cascade water_efficiency for Table 4b oil boiler (code
+  127, eff=84%) produces HW kWh 2785; worksheet 3639. Worksheet
+  effective HW eff ≈ 65% (suggests summer/winter blend or different
+  Appendix D path). Per SAP 10.2 Appendix D §D2.1 — the cascade may
+  not apply the right summer-eff override for non-PCDB oil boilers.
+- **solid fuel 2**: anthracite (eff=65%). Cascade HW is 3599 kWh;
+  worksheet HW likely lodged in a different line ref (the probe regex
+  returned 0). Re-probe needed.
+
+**Recommended approach**: take these as 3 separate per-variant slices
+in a fresh session. Each is its own diagnosis. The "+2.5" magnitude
+similarity is coincidence, not signal.
 
 ### 2. **pcdb 1 PE -3135 (single variant, biggest residual)** — open