docs: correct 2130/7536 drive-to-zero diagnosis (distinct causes, not shared)

Per each cert's notes: 7536 is a glazing-U gap (S0380.97 glazing_type=2
Table 24 default vs the cert's higher lodged U on multi-age bps) — the
tractable target; 2130's SAP +1 is a PV-β cohort cascade interaction, not
a fabric line. The earlier "multi-part wall, shared cause" label was wrong.

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

domain/sap10_calculator/docs/HANDOVER_MAPPER_BUGS.md

@@ -19,21 +19,28 @@ API-mapper/cascade bugs the audit surfaced.
 `_EXPECTATIONS` in `test_golden_fixtures.py` holds **53 pinned golden certs**. The suite
 is GREEN. **Only 3 have a non-zero SAP integer residual**, and they split into two kinds:
 
-| cert | lodged | cont SAP | resid | nature |
+| cert | lodged | cont SAP | resid | nature (from the cert's `notes:`) |
 |---|---|---|---|---|
-| **2130-1033-4050** | 82 | **83.349** | **+1** | REAL +0.85 over-prediction — multi-part **solid-brick** wall fabric |
-| **7536-3827-0600** | 68 | **69.071** | **+1** | REAL +0.57 over-prediction — multi-bp **D/L/F cavity** wall fabric |
-| **0240-0200-5706** | 73 | **72.462** | **−1** | **ARCHITECTURAL — do NOT chase, do NOT re-pin** (see below) |
+| **7536-3827-0600** | 68 | **69.071** | **+1** | +0.57 over — multi-age bps (Main D / Ext1 L / Ext2 F); **glazing U** (S0380.97 set glazing_type=2 → Table 24 spec U=2.0, but the cert's lodged U "appears higher than the spec default") |
+| **2130-1033-4050** | 82 | **83.349** | **+1** | +0.85 over — end-terrace + 1 ext, gas combi PCDB 17505, **2× PV arrays**; SAP +1 came from the **cohort PV-β cascade interaction** (S0380.45/.49), not a pinpointed fabric line; PE residual −8.22 sits in gas-combi PE + secondary credit |
 
-**The two real targets are 2130 + 7536.** Both are fabric *over*-predictions on **multi-part
-wall** geometry (a wall with alternative-construction sub-areas / multiple building parts),
-so they may share a single root cause in the §3 per-part wall area/U handling — a fix to
-one could close both. Method: walk the §3 heat-transmission cascade per-part vs the lodged
-register subsystem ratings (the certs are API-only — no worksheet — so the bar is ±0.5 SAP
-vs lodged, but the goal is the integer flip 83→82 / 69→68, i.e. shave ~0.85 / ~0.57 off the
-continuous SAP). If a multi-part-wall U or area is over-counted, that is the lever. The user
-generates Elmhurst worksheets on request — ASK for a simulated case mirroring 2130's wall
-(multi-part solid brick) rather than guess a U-value (the session methodology lesson).
+**These are TWO DIFFERENT root causes — not a shared one** (an earlier audit label calling
+both "multi-part wall" was wrong; trust the `notes:` above).
+
+- **7536** is the more tractable: a clear **glazing-U** hypothesis. S0380.97 forced
+  `glazing_type=2` to the Table 24 default U=2.0; the note suspects the cert's true per-bp
+  glazing U is higher (multi-age D/L/F geometry). Walk §3 window `(27)` per-bp vs the lodged
+  register window rating; the lever is likely the glazing U for one of the extensions. ASK
+  the user for a simulated Elmhurst worksheet mirroring 7536's glazing (double-glazed,
+  multi-age bps) to pin the true `(27)` U rather than guess.
+- **2130** is harder: the SAP +1 is a PV-β / cohort *cascade interaction*, not a single
+  fabric line. Its PE residual (−8.22) is a documented gas-combi-PE + secondary-credit
+  deferral. Decompose which metric drives the integer flip (cost/EI vs PE) before touching
+  anything; this one may need the PV/secondary path, not fabric.
+
+Both certs are API-only (no worksheet) → bar is ±0.5 SAP vs lodged; the goal is the integer
+flip (69→68 / 83→82), i.e. shave ~0.57 / ~0.85 off the continuous SAP. Per the session
+methodology lesson, ASK for a worksheet rather than guess a U-value or factor.
 
 **0240 (−1) is NOT driveable from the JSON and the user has decided NOT to re-pin it —
 document the cause only.** Continuous SAP is 72.462; the true SAP is **72**. The lodged