docs: session-4 handover — exposed-floor fix shipped, floor-3 enum unconfirmed

Record that profiler lead #1 (floor_codes=3) is not a clean single cause
(bimodal + confounded), that the paired worksheet certs confirm only
codes 1/6/7 (code 3 unmapped → needs a worksheet for 0380-2087), and that
immersion_type=2 / main_control=2107 / roof_codes=1 are scatter, not
dispatch bugs. The exposed-floor-on-flats fix (§3.12) shipped at b40e0f67.

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

docs/HANDOVER_API_PROFILING.md

@@ -39,6 +39,27 @@ profile-surfaced buckets below.
    (4)(5)(6) cleared **all 4 raises** — eval now has zero raises.
 7. `(profiler)` **`scripts/profile_api_error.py`** — the new diagnostic (below).
 
+## SESSION-4 UPDATE (HEAD `b40e0f67`) — read before re-working the leads below
+- **Lead #1 `floor_codes=3` is NOT clean — worked it, enum UNCONFIRMED.** It's bimodal
+  (mid-floor flats over-rate, top-floor maisonettes 0434/0761 want ~zero floor loss) and
+  confounded (9763 +19.48 is a WALL bug: walls=8.19 W/K for 59.8 m²). The 48 paired
+  API+Summary worksheet certs (`sap worksheets/Additional data with api/` + `additional with
+  api 2/`, folder=API cert id) confirm **code 7↔"G Ground", code 1↔"E To external air",
+  code 6↔no-heat-loss** but **NONE cover code 2 or 3** → code 3 is genuinely unmapped.
+  RdSAP §3.12 (p.25) flat floor categories: exposed→Table20, semi-exposed(unheated)→Table20,
+  above-partial(non-domestic)→0.7, ground→ISO13370. **NEXT: get a worksheet for
+  `0380-2087-8190-2996-3075`** (mid-floor flat, single BP, roof correctly party so the floor
+  is the ONLY heat-loss unknown; lodged 66, we +3.71). Tried Table-20 for codes 2/3: overshoots
+  (9494 +0.56→-3.67), reverted — and picking ground-U-vs-Table20 by eval score is a data-fit.
+- **SHIPPED `b40e0f67`:** exposed-floor-on-flats (code 1) area fix — §3.12. A flat's code-1
+  floor was area-zeroed by `_dwelling_exposure`; now the per-BP `is_exposed_floor` overrides the
+  flat suppression upward (mirrors the "another dwelling below" party override). 45.1→45.3%.
+- **Leads re-checked, NOT clean:** `immersion_type=2` (+1.86) is high-scatter (mean|err| 3.71,
+  bidirectional). `main_control=2107` (+1.63) is correctly mapped ("Programmer, TRVs and bypass"
+  type 2 Table 4c(2)) — over-rate is diffuse gas-boiler/flat-fabric, not a dispatch bug.
+  `roof_codes=1` broad bucket is mean −0.15 (the −1.78 was top-floor-electric-flat outliers
+  −29/−25). Remaining gains need per-cert worksheets (start code-3) or the unsupported-schema ticket.
+
 ## KEY INSIGHT (load-bearing, from the user)
 **The gov EPC API JSON is the published OUTPUT of RdSAP software (Elmhurst), not its input.**
 So any API field Elmhurst doesn't expose as an *input* is register metadata the RdSAP10 method