Handover: hand-built fixture skeleton landed (Slice 62); 2/11 pins green

Update NEXT_AGENT_PROMPT.md with the pivot to the rigorous cohort
pattern: cert 001479's hand-built `_elmhurst_worksheet_001479.py`
becomes the ground-truth EpcPropertyData. Cross-mapper parity work
then collapses to "both mappers produce hand-built-equivalent
EpcPropertyData".

Two parallel workstreams documented:

1. Iterate the hand-built skeleton (Slice 62) until all 11 cascade
   pins hit 1e-4. Current state: 2/11 green (pumps_fans, lighting);
   sap_score_continuous gap −3.02 SAP. Likely next slices: HW demand
   routing, §2 ventilation tuning, thermal mass parameter, multiple-
   glazed proportion.

2. Once hand-built is GREEN, add `test_elmhurst_mapper_matches_hand_
   built` + `test_api_mapper_matches_hand_built` over the 7-cert
   cohort (000474..000516 + 001479). Every field diff = mapper bug
   to close. Cross-parity collapses to "both mappers produce
   hand-built-equivalent".

Documents the M-vs-L Ext1 age-band source-data conflict (hand-built
uses worksheet's L; Elmhurst mapper trusts Summary's M) — surfaces
as a known caveat in cross-mapper diff.

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

docs/sap-spec/NEXT_AGENT_PROMPT.md

@@ -232,7 +232,9 @@ For the new workflow, you'll want a probe that:
 ## Branch state at handover
 
 ```
-$ git log --oneline -10
+$ git log --oneline -12
+ee98dbe0  Slice 62: hand-built _elmhurst_worksheet_001479.py — skeleton + 11 RED pins
+0e4f4c05  Handover: TDD red-green session — 4 more slices (58-60) + RED chain pin
 31c01a7e  Slice 60: thermal bridging y is dwelling-wide, not per-bp
 175873b4  Slice 59: heat_transmission apportions window area per bp via window_location
 e3dc0b28  Slice 58: secondary fuel cost routes through lodged secondary_fuel_type
@@ -356,14 +358,65 @@ chain-test fixtures dir.
 
 ### Suggested next steps
 
-1. **`u_floor` Table 19 audit for age C suspended timber** — verify
-   the cascade matches the canonical RdSAP10 / xlsx. Possible fix
-   without breaking cohort 000474 (which is age B).
-2. **PS sloping-ceiling roof area inclination factor** — multiply
-   `top_floor_area_m2` by `sec(pitch)` when roof_type is PS. Closes
-   the (30) and (31) gaps simultaneously.
-3. When the chain pin is within ~0.001 of 69.0094, the RED test goes
-   GREEN — workstream done. Then API ±0.5 pin (test_golden_fixtures
-   _.py) and cross-parity test are the remaining workstreams.
+**User-confirmed plan (rigorous cohort pattern):** the hand-built
+fixture `_elmhurst_worksheet_001479.py` (Slice 62) is the ground-
+truth EpcPropertyData for this cert. Two parallel workstreams now:
+
+1. **Iterate the hand-built to 1e-4 against the worksheet.** Current
+   state: 2/11 cascade pins green (pumps_fans, lighting after the
+   LED/CFL split). The other 9 pins fail with `sap_score_continuous
+   = 65.99 vs 69.0094` (~3 SAP gap). Likely slice candidates from
+   the cascade scalar deltas:
+   - **HW demand routing**: hand-built may be over-counting hot-
+     water demand (combi-vs-cylinder path; Tcold model; Appendix J
+     occupancy). The worksheet's `(219) 2358.31` vs cascade's
+     `hot_water_kwh_per_yr` is one of the highest-impact deltas.
+   - **§2 ventilation tuning**: confirm `open_chimneys_count=0`,
+     `blocked_chimneys_count=0`, `closed_flues_count=0`,
+     `passive_vents_count=0` are all explicitly lodged on
+     `SapVentilation` to match the worksheet's §2 zeros.
+   - **Thermal mass parameter**: worksheet lodges `250.00` — verify
+     the hand-built's default matches.
+   - **`multiple_glazed_proportion`**: cascade reads it for solar
+     gain weighting; hand-built leaves None — check if that path
+     short-circuits to a less-favourable default.
+   - **`secondary_heating_fraction`**: cascade may be reading 0.10
+     (gas+gas) vs Elmhurst's 0.10 — confirm. (215) delta is ~290
+     kWh; worth ~0.2 SAP if mis-routed.
+
+2. **Once 11 pins green: add `test_elmhurst_mapper_matches_hand_
+   built` + `test_api_mapper_matches_hand_built`** parametrized
+   over both the new cert 001479 and the 6 cohort certs. Every
+   field diff is a mapper bug; close them slice-by-slice. The
+   cross-mapper parity test (`test_api_vs_elmhurst_parity`)
+   collapses to "both produce hand-built-equivalent EpcPropertyData
+   for cert 001479".
+
+3. **Current Elmhurst chain pin** (`test_summary_001479_full_chain
+   _sap_matches_worksheet_pdf_exactly`) is RED at delta 1.19 SAP.
+   Once the mapper closes its diff vs the hand-built, the chain pin
+   lands GREEN automatically.
+
+### Probe scripts in /tmp (regenerable)
+
+- `/tmp/probe_001479.py` — cross-mapper diff + cascade (rerun
+  after every cascade change; current diff count: 215 across both
+  mappers).
+- `/tmp/sensitivity_001479.py` — single-field SAP impact probe.
+- `/tmp/perbp_001479.py` — per-bp cascade U-value dump.
+
+### Cohort cascade scalars probe (helpful for hand-built iteration)
+
+Pin the failing fields against an MCVE probe — easiest workflow:
+
+```python
+from domain.sap.calculator import Sap10Calculator
+from domain.sap.worksheet.tests._elmhurst_worksheet_001479 import build_epc
+r = Sap10Calculator().calculate(build_epc())
+# Inspect r.hot_water_kwh_per_yr, r.main_heating_fuel_kwh_per_yr, etc.
+```
+
+Compare against the cohort cert (e.g. 000490 mains-gas+gas secondary)
+to find what hand-built field is missing.
 
 Good luck.