docs: handover post S0380.189 — TMP/Table 22 + the two open follow-ups

Point-in-time note for the next agent: what S0380.185-189 shipped (worksheet
PE/CO2 pins, the two D_PV electricity-vs-gain fixes, and the thermal-mass-
parameter Table 22 fix), the per-line diagnosis template, the two worksheet-
block / gains-vs-solar traps, and the ranked open slices (Summary-path fuel
derivation first, then pin the simulated 001431 case, then cert 6035).

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

domain/sap10_calculator/docs/HANDOVER_POST_S0380_189.md

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+# Handover — post S0380.189 (thermal mass parameter / Table 22)
+
+Point-in-time note. Start from [`AGENT_GUIDE.md`](AGENT_GUIDE.md) for the
+methodology, accuracy bar, and pipeline — this doc only records *what this
+session did* and *what is open*.
+
+- **Branch:** `feature/per-cert-mapper-validation`
+- **HEAD:** `e03f08cd` (S0380.189)
+- **Baseline:** `2290 passed, 1 skipped, 0 failed` (the skip is the
+  gitignored xlsx `load_cells` test). Verify with the §4 suite command.
+
+---
+
+## What this session shipped (S0380.185–189)
+
+| Slice | What | Spec |
+|---|---|---|
+| **.185** | Recorded the CH6 "pin-forever" proof — distribution-loss is an Elmhurst Summary-export gap, not a mapper miss (controlled adjoining-dwellings pair byte-identical in inputs). | — |
+| **.186** | Added `test_golden_cert_pe_co2_matches_worksheet` — pins calc PE/CO2 for the 47 worksheet-backed certs against the dr87 `(286)`/`(272)` at full precision (not lodged register values). | Appendix U |
+| **.187** | Appendix M1 §3a `D_PV,m` was missing **electric secondary** space heating `(215)m` — under-credited PV on gas-main+electric-secondary+PV certs. | SAP 10.2 App M1 §3a |
+| **.188** | `D_PV,m` used the Appendix L **L12 lighting GAIN** (`= E_L×0.85`) instead of the **L10 lighting ELECTRICITY** `(232)`. Closed the whole PV cohort to 1e-4. Same gain-vs-electricity class as the S0380.73 cooking fix. | SAP 10.2 App L L10/L12, M1 §3a |
+| **.189** | **Thermal mass parameter** was hardcoded 250 at all 5 §7/§8 call sites. Now `_thermal_mass_parameter_kj_per_m2_k(epc)` per Table 22. | RdSAP 10 §5.16 Table 22 (p.48) |
+
+After .186–.188 **all 47 worksheet-backed certs match calc≡worksheet at
+<1e-4 on PE and CO2** (the convergence target). See
+[`project-golden-coverage-state` memory] for the per-slice detail.
+
+---
+
+## The S0380.189 diagnosis (template for the open work)
+
+Driven entirely by the **per-line walk** (AGENT_GUIDE §3) against a
+**user-simulated worksheet** for a 6035-archetype property:
+
+- **Fixture:** `sap worksheets/golden fixture debugging/simulated case 1/`
+  — `Summary_001431 (1).pdf` (input) + `P960-0001-001431 - ….pdf` (worksheet
+  ground truth). A gas-combi mid-terrace, TFA 128, solid brick **with
+  internal insulation**, no PV/secondary/cylinder. (The `P960-…` prefix is
+  just the Elmhurst account id; cert is 001431.) **These PDFs are untracked**
+  — the user holds them locally.
+- **Decompose:** PE +8.78 / SAP −1.76 / CO2 +0.21, *entirely* space-heating
+  demand (+838 kWh). Fabric `(26–37)`, internal gains `(73)`, climate
+  `(96)m`, HTC `(39)` all EXACT → localised to **§7 MIT** `(92)` +0.71 °C.
+- **Root cause:** TMP hardcoded 250; cert is masonry **with internal
+  insulation** → RdSAP 10 §5.16 Table 22 = **100**. Wrong TMP → time
+  constant τ=Cm/(3.6·H) ≈40 h not 16 h → §7 temperature reduction too small
+  → MIT too high → space heating over-stated.
+- **Fix + blast radius:** only golden cert **6035** re-pinned (SAP −6→−2, PE
+  +46.42→+19.16, CO2 +1.07→+0.42). All other fixtures are masonry-no-internal
+  → TMP unchanged.
+
+### Two diagnostic traps that cost false starts (read before debugging §7/§8)
+
+1. **The worksheet has TWO blocks per dwelling.** The first is the SAP-rating
+   block (UK-average climate, region 0); the second, under *"CALCULATION OF
+   EPC COSTS, EMISSIONS AND PRIMARY ENERGY"*, is the postcode block. The
+   **demand cascade (`cert_to_demand_inputs`) matches the POSTCODE block.**
+   Comparing calc HTC/MIT to the UK-avg block shows phantom gaps (e.g. HTC
+   "−10.8"); against the postcode block HTC is exact.
+2. **`(84)` Total gains = internal `(73)` + solar `(83)`.** The calc's
+   `internal_gains_annual_avg_w` is `(73)` only — don't diff it against `(84)`
+   (a phantom "−248 W" gap that is just solar).
+
+Use the §2.4 section helpers (`mean_internal_temperature_section_from_cert`,
+`space_heating_section_from_cert`, `internal_gains_section_from_cert`,
+`local_climate_for_cert`) for the per-line walk.
+
+---
+
+## OPEN — next slices (ranked)
+
+### 1. Summary-path `main_fuel_type` derivation from the SAP code  ← do first
+The Elmhurst **Summary PDF has no main-heating fuel field** — only the SAP
+code (`14.0 Main Heating1 → Main Heating SAP Code 104`). So
+`from_elmhurst_site_notes` leaves `main_fuel_type=''`, and `cert_to_inputs`
+raises `MissingMainFuelType` (cert_to_inputs.py `_main_fuel_code`). This
+**blocks the Summary path for every gas-combi cert**, including the simulated
+case (I injected `main_fuel_type=26` to run the diagnosis).
+
+Fix: derive the fuel in the mapper (or `_main_fuel_code`) from
+`sap_main_heating_code` via SAP 10.2 Table 4b (104 = condensing combi **mains
+gas**), mirroring the existing strict-raise → derive pattern. Cite the table.
+This is the higher-leverage win — it unblocks the whole site-notes gas-combi
+population, not one cert.
+
+### 2. Pin the simulated 001431 case end-to-end (after #1)
+Once #1 lands, the Summary path runs natively. Add it as a site-notes fixture
+and pin every line ref at 1e-4. **NB:** with .189 + injected fuel, PE and CO2
+close to 1e-4 but **SAP showed +0.0007** vs a hand-computed target (`100 −
+13.95×ECF` from the 4-dp `(257)`=1.6047). That is almost certainly ECF
+rounding in the target, not a real gap — but **verify against the worksheet's
+own continuous SAP** before declaring it closed (don't trust my hand value).
+
+### 3. Cert 6035 remaining +19 PE
+6035 is API/lodged-only (no worksheet) so it can't be pinned past the lodged
+register. The user can reproduce 6035 *exactly* in Elmhurst to get a
+worksheet — offer to format the golden JSON
+(`tests/domain/sap10_calculator/rdsap/fixtures/golden/6035-7729-2309-0879-2296.json`)
+as Elmhurst inputs. With a worksheet, walk the remaining +19 the same way.
+
+### Carry-over (lower priority)
+- `transform.py:973` treats `wall_construction in (5,6)` as timber-frame for
+  the ventilation structural-ACH split, but Table 22 / `rdsap_uvalues`
+  classify 6 = **system built (masonry)**, only 5/7/8 are timber/cob/park.
+  Possible latent ventilation-ACH bug — verify before touching.
+
+---
+
+## Process notes
+- One slice = one commit, spec citation in the message, `Co-Authored-By:
+  Claude Opus 4.8` trailer. AAA tests, `abs(x-y) <= tol` (not `pytest.approx`).
+- The golden worksheet PE/CO2 pins (.186) re-pin smaller as gaps close — never
+  widen. The lodged-residual pins (`test_golden_cert_residual_matches_pin`)
+  carry the API-vs-register residual and move when the calc improves.