docs: handover — fold in S0380.232-234 (case-19 PV closure) + open causes

Records the three PV slices shipped (D_PV off-peak exclusion, weighted
dwelling import price, Appendix G4 diverter), the resulting case-19 state
(SAP 50.33→51.34, rounds to lodged 51), and the two remaining case-19
causes (winter Appendix-M EPV monthly shape; fabric (33) +1.0). Adds the
`2100-5421` worst-offender diagnosis (a 352 m² uninsulated solid-wall
dwelling on the as-built-insulated-assumed roof-U front, not a flats bug).

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

domain/sap10_calculator/docs/HANDOVER_API_SAMPLE_ACCURACY.md

@@ -4,11 +4,43 @@ Point-in-time note. Start from [`AGENT_GUIDE.md`](AGENT_GUIDE.md) for methodolog
 1e-4 bar, the per-line debugging loop, the section helpers, and the suite command.
 
 - **Branch:** `feature/per-cert-mapper-validation`
-- **HEAD:** `f326e4eb`. Next SAP slice: **S0380.232**.
+- **HEAD:** `9521d524`. Next SAP slice: **S0380.235**.
 - **Baseline (§4 suite):** `tests/domain/sap10_calculator/ backend/documents_parser/tests/`
-  → green (2407 passed, 1 skipped). Pre-existing out-of-scope failures unchanged
+  → green (2412 passed, 1 skipped). Pre-existing out-of-scope failures unchanged
   (stone-§5.6 in `domain/sap10_ml/tests/`; `test_from_rdsap_schema.py::...test_total_floor_area`).
 
+## Shipped this session (S0380.232-234 — the case-19 PV closure)
+
+The PV diverter (the prior handover's S0380.232 ask) needed two prerequisite
+spec bugs fixed first; all three landed:
+
+| slice | commit | spec | what |
+|---|---|---|---|
+| **S0380.232** | `212b0c92` | App M1 §3a (p.93, l.5470-5476) | D_PV excludes the LOW-rate portion of an off-peak electric main: `(211)` is only PV-eligible where its §10a code ∈ {30,32,34,35,38}. Storage heaters on 7-hr charge wholly at low rate → fraction 0.0 → excluded. β_Jan 0.894→0.792 (ws 0.791). New `_main_space_heating_high_rate_fraction`. |
+| **S0380.233** | `d4a8c02b` | App M1 §6 (p.94, l.5510-5513) | PV-used-in-dwelling credited at the Table 12a ALL_OTHER_USES **weighted** rate (7-hr 14.311 p/kWh), not the bare low rate (5.50). Was under-crediting onsite PV on every off-peak PV cert. Delegates to `_other_fuel_cost_gbp_per_kwh`; STANDARD unchanged. |
+| **S0380.234** | `9521d524` | Appendix G4 (p.72-73) | The PV diverter. 3 layers: extractor `Diverter present` + schema `pv_diverter` → `pv_diverter_present` flag (Elmhurst + API mappers) → `_pv_diverter_monthly_kwh` (SPV = export×0.8×0.9, clamp ≤ (62)+(63a), → (63b)m); `cert_to_inputs` recomputes (219) + PV export, β fixed pre-diverter. |
+
+**Case 19 now: SAP cont 50.33 → 51.34** (ws 51.2221; both round to lodged **51**),
+cost (255) 1847.5→1812.3 (ws 1816.6), CO2 3331→3120 (ws 3126), (233a) dwelling
+1280.6 (ws **1280.4** — the β fix pins it). The diverter formula is **exact in
+summer** (Jun SPV 186.07 = export×0.72, matches ws (63b)).
+
+**The remaining +0.11 SAP on case 19 = two separate, still-open causes:**
+1. **Winter Appendix-M monthly EPV shape.** Our annual EPV (2684.17) matches the
+   worksheet exactly and Jun-Sep match per-month exactly, but Jan-May/Oct-Dec our
+   EPV is ~9-11% LOW (worksheet Jan 68.2 vs ours 62.5). Back-solve: ws EPV_m =
+   |(233a)_m| + |(63b)_m|/0.72. This under-diverts in winter → export (233b) 280.7
+   vs ws 184.2, and (219) 3322 vs ws 3188. **A two-array PV apportionment issue
+   (case 19 has SE + NW arrays with different overshading) — chase in §M / Appendix U
+   monthly radiation, NOT the diverter (which is validated).**
+2. **Fabric (33) +1.0 W/K** (ours 305.04 vs ws 304.04) — a single element off by
+   exactly 1.0; floor=25.000 is suspiciously round. Walk the per-element §3 breakdown.
+
+The **eval headline is flat** (42.9→43.0% <0.5; cat-7 5.25→4.93) — expected: the
+diverter is rare and the β/price effects are small on the rounded SAP. The value
+was pinning the worksheet-validated case 19 + fixing three real spec bugs that the
+curated cohort masked.
+
 ## Headline now (1,000-cert 2026 API sample, HEAD `f326e4eb`)
 
 | metric | value | was (handover baseline `9c0a373f`) |
@@ -188,6 +220,14 @@ pages → ElmhurstSiteNotesExtractor(...).extract() → from_elmhurst_site_notes
 4. **Non-PCDB gas boilers (cat 2, no idx, 91 certs, mean 3.18)** and **Flats (282, mean 2.57)** —
    the next volume levers once the electric clusters are worksheet-pinned. Flats = geometry /
    communal; start with the worst (`2100-5421` negative SAP).
+   - **`2100-5421-0922-1622-3463` diagnosed (S0380.234 session):** NOT a flat — `property_type 0`,
+     a **352 m² 2-storey uninsulated solid-wall** dwelling (wall_constr 3 / wall_ins 4 as-built;
+     roof_type 4, no roof insulation). Our space-heating demand is **71,084 kWh/yr** → (37)=995.93
+     W/K → SAP −24.8 (lodged 36), cost £14,045. This is the **`as-built insulated-assumed`**
+     U-value front ([[project_as_built_insulated_assumed_bug]]; S0380.209 fixed walls, "roof next"):
+     the uninsulated-roof / as-built U over-estimates demand on big old dwellings. API-only (no
+     worksheet → ±0.5 lodged fallback only); needs a generated worksheet or a roof-U spec audit to
+     pin. It is one outlier, not a cluster-wide flats bug.
 
 ### B. Remaining raises (16 certs — all U-value / heat-loss-sensitive, NOT enum guesses)
 - **`gable_wall_type` 2 & 3 (14 certs).** RdSAP 10 **Table 4** RR walls: 0=Party (U=0.25),