Handover: TDD red-green session — 4 more slices (58-60) + RED chain pin

Update NEXT_AGENT_PROMPT.md for the TDD session that landed 3 more
slices on top of Session 1's fabric work:

  58: secondary fuel cost routes through lodged secondary_fuel_type
      (closes the biggest single gap on cert 001479 — 9 SAP)
  59: heat_transmission apportions windows per bp via window_location
  60: thermal bridging y uses primary bp's age (dwelling-wide)

Chain pin `test_summary_001479_full_chain_sap_matches_worksheet_pdf_
exactly` is committed RED as the load-bearing TDD forcing function:

  Pre-workstream: delta +5.84 SAP (cascade 63.17 vs target 69.0094)
  Post-Slice 60: delta −1.19 SAP (cascade 70.20 vs target 69.0094)

Per-bp fabric U-values all match the worksheet exactly. Remaining
1.19 SAP overshoot maps to ~3 W/K of HLC undercount in roof + floor:

- Ext2 PS sloping-ceiling roof area uses floor projection (1.92 m²)
  instead of slant area (2.22 m²). −0.81 W/K.
- Main ground-floor U: `u_floor` Table 19 returns 0.60 for age C;
  worksheet expects 0.65 (same as age B). −1.52 W/K.
- (31) external area under-count drives bridging gap. −2.08 W/K.

Slice 61 (SapFloorDimension.floor_lodged_u_value override using
Summary §9 "Default U-value") was attempted and reverted: closed
001479 floor gap exactly but broke 000474 cohort's 1e-4 pin (its
cascade calibration uses u_floor age-B 0.77 vs Summary's lodged
0.75). Next session needs a different fix — Table 19 audit for
age C, or selective override.

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

docs/sap-spec/NEXT_AGENT_PROMPT.md

@@ -232,126 +232,138 @@ For the new workflow, you'll want a probe that:
 ## Branch state at handover
 
 ```
-$ git log --oneline -8
+$ git log --oneline -10
+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
+a0d9d094  Handover: 4 cert-001479 slices in (54-57); gap at +7.62 SAP; non-fabric next
 7a9a8b7e  Slice 57: Pre-1950 Elmhurst sloping-ceiling roofs map to thickness=0
 07ed871f  Slice 56: Elmhurst floor exposed to external air routes through u_exposed_floor
 c89206fc  Slice 55: Elmhurst party-wall code "CU" maps to cavity unfilled
 4427b58a  Slice 54: Elmhurst mapper sets extensions_count from len(survey.extensions)
 a756114a  Handover: all 6 Elmhurst Summary→SAP chains closed at 1e-4
 58088c10  Slice 53: Summary_000487 chain pins SAP at 1e-4 — last cohort cert closed
-4ccf9c97  Slice 52: Summary_000477 chain pins SAP at 1e-4; electric shower + decimal RIR rounding
-cb4e31a1  Slice 51: Summary_000516 chain pins SAP at 1e-4; roof-window separation
 ```
 
-758 cohort + cert-001479 structural tests green; pyright net-zero
-(35 baseline) on touched files.
+Chain pin `test_summary_001479_full_chain_sap_matches_worksheet_pdf_
+exactly` is committed RED (cascade SAP 70.20 vs worksheet 69.0094,
+delta 1.19) as the load-bearing TDD forcing function. All other
+chain + golden + heat-transmission tests pass. Pyright net-zero on
+touched files.
 
-## Resumption notes for cert 001479 (Slices 54–57 partial progress)
+## Resumption notes for cert 001479 (Slices 54–60 in; chain pin RED at delta 1.19)
 
-### What landed
-Four mapper slices closed real Elmhurst-side gaps that surfaced when
-the cross-mapper diff against the new API counterpart was run for
-the first time:
+### What landed across two sessions
 
-- **Slice 54** — `extensions_count` now reads `len(survey.extensions)`
-  instead of the hard-coded `0`. No SAP impact (the cascade iterates
-  `sap_building_parts`), but a real correctness fix the cross-mapper
-  parity assertion needs.
-- **Slice 55** — Elmhurst party-wall code `"CU"` (Cavity masonry
-  unfilled) now maps to SAP10 `WALL_CAVITY=4`; `u_party_wall` returns
-  0.5 W/m²K matching the worksheet's lodged `Party walls Main … 0.50`.
-- **Slice 56** — Floor location `"E To external air"` now routes
-  through `u_exposed_floor` (Table 20), matching cert 001479 Ext2's
-  cantilevered exposed timber floor at U=1.20.
-- **Slice 57** — PS (Pitched, sloping ceiling) roofs with no lodged
-  thickness ("As Built") and age band A-D map to `thickness=0`,
-  giving Table 16 row-0 U=2.30 — matches cert 001479 Ext2's worksheet
-  `External roof Ext2 … 2.30`. Ext1 (age M) keeps thickness=None
-  → cascade default 0.15.
+**Session 1** (Slices 54-57): fabric mapper gaps from the cross-mapper diff.
+
+- **Slice 54** — `extensions_count` reads `len(survey.extensions)`.
+- **Slice 55** — Elmhurst party-wall code `"CU"` → `WALL_CAVITY=4`
+  (U=0.5 matching worksheet's `Party walls Main … 0.50`).
+- **Slice 56** — Floor location `"E To external air"` routes through
+  `u_exposed_floor` (Ext2 cantilevered floor at U=1.20).
+- **Slice 57** — PS sloping-ceiling roofs at age A-D with "As Built"
+  thickness map to `thickness=0` → U=2.30 (Ext2 uninsulated roof).
+
+**Session 2** (Slices 58-60): TDD red-green cycle with the chain pin as
+forcing function. Two cascade-level fixes + one mapper fix:
+
+- **Slice 58** — Secondary fuel cost routing. Mapper derives
+  `secondary_fuel_type=26` (mains gas) from SAP code 605; cascade
+  `_fuel_cost` reads `secondary_fuel_type` instead of hardcoding the
+  electric tariff. Closes a £175/yr ECF distortion ≈ **9 SAP** on
+  cert 001479. Golden cert 0300-2747 (also mains-gas secondary)
+  tightens SAP residual −7 → +2 — biggest single golden improvement.
+- **Slice 59** — `heat_transmission_from_cert` apportions window
+  area per `window_location` to each bp's wall deduction (was all-to-
+  Main). For 001479 Ext1's 6.37 m² window now correctly cuts into
+  Ext1's wall (U=0.26) instead of Main's (U=0.70). Three golden
+  certs (6035, 7536, 8135) with non-Main windows tighten all
+  residuals; cohort certs unaffected (uniform per-bp wall U).
+- **Slice 60** — Thermal bridging `y` is dwelling-wide (primary bp's
+  age band) rather than per-bp. Multi-age dwellings like 001479
+  (Main=C, Ext1=M, Ext2=C) and golden 7536 (D, L, F) had Ext1
+  bridging under-counted at y=0.08 instead of dwelling's y=0.15.
+
+**Slice 61 ATTEMPTED + REVERTED**: `SapFloorDimension.floor_lodged_
+u_value` override using Elmhurst Summary §9 "Default U-value". The
+override matched 001479's worksheet exactly (Main 0.65, Ext1 0.20,
+Ext2 1.20) but broke cohort 000474's 1e-4 pin: that cert's cascade
+calibration relied on `u_floor` returning 0.77 for age B + 12.68 m²,
+while Summary lodges 0.75. The 0.02 U drift × 12.68 m² shifted SAP
+beyond 1e-4. **Next session needs a different approach** — either
+fix `u_floor` Table 19 cascade for age C (currently 0.60, should be
+0.65) without breaking age B, or selectively apply the override.
 
 ### Where the chain stands
 
-Mapped Elmhurst cascade for cert 001479:
-- Pre-Slice 54: SAP 63.17 vs worksheet 69.0094 (gap +5.84)
-- Post-Slice 57: SAP **61.39** vs worksheet 69.0094 (gap **+7.62**)
+| Cascade SAP | Delta to 69.0094 | After |
+|---|---|---|
+| 63.17 | +5.84 | Initial (pre-this-workstream) |
+| 61.39 | +7.62 | Post-Slice 57 (fabric only) |
+| 70.64 | −1.63 | Post-Slice 58 (secondary fuel) |
+| 70.38 | −1.37 | Post-Slice 59 (window apportionment) |
+| **70.20** | **−1.19** | **Post-Slice 60 (single-y bridging)** |
 
-Gap widened because each fix was per-data correct; the previous
-mapper state was under-counting fabric heat loss in multiple places
-that were collectively offsetting some over-counting elsewhere.
-Per-bp wall U-values now all match the worksheet exactly:
+The chain pin is committed RED at delta 1.19. **Per-bp fabric U-values
+all match worksheet exactly** (Main wall 0.70, Ext1 wall 0.26, Ext2
+wall 0.70, etc.). The remaining 1.19 SAP overshoot maps to ~3 W/K of
+extra HLC that the cascade is still under-counting:
 
-| BP | Wall | Roof | Party | Floor |
-|---|---|---|---|---|
-| Main | 0.70 ✓ | 0.14 ✓ | 0.50 ✓ | 0.65 (ground cascade) |
-| Ext1 | 0.26 ✓ | 0.15 ✓ | n/a (pwl=0) | 0.20 |
-| Ext2 | 0.70 ✓ | **2.30 ✓** | n/a (pwl=0) | **1.20 exposed ✓** |
+| Line ref | Cascade | Worksheet | Gap |
+|---|---|---|---|
+| (29a) walls | 39.77 | 39.77 | ✓ |
+| (30) roof | 9.53 | 10.34 | −0.81 (Ext2 sloping-ceiling area) |
+| (28a) floor | 21.65 | 23.17 | −1.52 (Main floor U 0.60 vs 0.65) |
+| (32) party | 17.07 | 17.07 | ✓ |
+| (27) windows | 43.60 | 43.60 | ✓ |
+| (26) doors | 5.55 | 5.55 | ✓ |
+| (36) bridging | 22.27 | 24.35 | −2.08 (driven by (31) under-count) |
+| **(37) total** | **156.62** | **163.84** | **−7.22 W/K** |
 
-Fabric is essentially complete. The remaining ~7.6 SAP gap lives in
-non-fabric inputs.
+### What likely closes the remaining 1.19 SAP
 
-### What likely drives the remaining 7.6 SAP
+1. **`u_floor` Table 19 boundary for age C** (cascade returns 0.60;
+   worksheet expects 0.65 — same as age B). May be a Table 19 row
+   boundary miss. Need to read the canonical xlsx Sheet `Table 19`
+   to confirm correct values. If cascade is wrong, fixing it would
+   affect cohort but probably in the right direction.
+2. **Ext2 roof area for PS sloping ceiling** — cascade uses floor
+   area (1.92) as roof area; worksheet uses 2.22 (slant length ×
+   width). Factor ≈ 1.156 = sec(30°). Cascade-level: multiply
+   gross_roof_area by an inclination factor when roof_type starts
+   with "PS".
+3. **`(31)` total external area under-count** of 1.13 m² (drives the
+   bridging gap). Probably the same Ext2 roof area issue (0.30 m²)
+   plus other accumulations. Fix #2 likely closes most of this.
 
-- **HLP check**: worksheet `HLP (average) 3.1269`; cascade
-  total_w_per_k / TFA = 153.15 / 68.51 = **2.235** — cascade is
-  under-counting total heat loss by ~61 W/K. Combined with cascade
-  ventilation HLC (~46 W/K) gives total ~199 vs worksheet's expected
-  ~214 — gap ~15 W/K in non-fabric.
-- **Living area fraction**: cascade `0.25`, worksheet `0.28`. The
-  worksheet computes 17.13/61.18 (Main TFA only?) vs cascade's
-  17.13/68.51 (all bp TFA). SAP convention question — may need
-  cascade-level fix.
-- **Internal gains**: cascade `lighting_kwh_per_yr=163` looks low for
-  23 fittings; cascade `pumps_fans_kwh_per_yr=160` may differ from
-  worksheet (which lodges main_heating_category=1).
-- **Secondary heating**: cascade has fraction=0.1, η=0.4 (matches
-  worksheet). SAP code 605 (gas fire flush, sealed-flue) is not wired
-  through; the cohort 000490 sets `secondary_heating_type` to a SAP
-  code int — verify cert 001479 needs the same.
-- **PCDB boiler index 17507 (Worcester Greenstar 30i)** — cascade
-  reads `main_heating_efficiency=0.89` (matches worksheet's 89%
-  winter) so likely already resolved. Confirm.
-- **Per-window U vs avg-U routing**: cascade takes per-window U path
-  (every window has `window_transmission_details`). Worksheet's
-  windows use 2.80 U (default) — verify cascade matches.
-
-### Source-data caveats found this session
+### Source-data caveats
 
 - **Summary PDF vs worksheet age band on Ext1**: Summary §3 says
   `M 2023 onwards`; worksheet header says `Property Age Band C, Ext1: L,
-  Ext2: C`. Likely assessor data-entry inconsistency. User decision:
-  trust the Summary PDF (M); accept whatever residual the worksheet's
-  L-based calc leaves. Document the caveat in the chain pin docstring
-  when it lands.
-- **Worksheet "0.0" Type column on External Walls**: looks like an
-  unused column header in Elmhurst's tabular output, not a
-  shelter-factor input. Cascade ignores it correctly.
+  Ext2: C`. Trust Summary (mapper does what data says); chain pin
+  docstring documents the caveat.
 
 ### Probe scripts in /tmp (regenerable)
 
-- `/tmp/probe_001479.py` — cross-mapper diff + cascade for both
-  mappers; baseline for comparing API vs Elmhurst EpcPropertyData.
-- `/tmp/sensitivity_001479.py` — single-field patch SAP impact probe;
-  useful for sequencing slices but stale after each commit (re-run).
-- `/tmp/perbp_001479.py` — per-bp cascade U-value dump vs worksheet
-  expected values; the cleanest "is fabric matching?" check.
+- `/tmp/probe_001479.py` — cross-mapper diff + cascade.
+- `/tmp/sensitivity_001479.py` — single-field SAP impact probe.
+- `/tmp/perbp_001479.py` — per-bp cascade U-value dump vs worksheet.
 
-Cached cert JSON is at `packages/domain/src/domain/sap/rdsap/tests/
-fixtures/golden/0535-9020-6509-0821-6222.json` (token-fetched once,
-no further API calls needed). Summary PDF copied into the chain-test
-fixtures dir.
+Cached cert JSON: `packages/domain/src/domain/sap/rdsap/tests/
+fixtures/golden/0535-9020-6509-0821-6222.json`. Summary PDF in the
+chain-test fixtures dir.
 
 ### Suggested next steps
 
-1. Probe the cascade's section-by-section line refs (§3 walls, §3
-   roofs, §3 windows, §3 thermal bridging, §2 ventilation HLC) against
-   the worksheet text to find the ~15 W/K HLC gap.
-2. Check `living_area_fraction` SAP convention — Main-only vs whole-
-   dwelling TFA. Cohort certs may have been single-bp so this
-   convention difference didn't surface.
-3. Wire secondary heating SAP code through if a §14 worksheet line ref
-   shows a different secondary contribution than the cascade.
-4. When the chain SAP is within ~0.1 of 69.0094, land the
-   `test_summary_001479_full_chain_sap_matches_worksheet_pdf_exactly`
-   pin at 1e-4 — that's the forcing function for the workstream.
+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.
 
 Good luck.