docs(adr): ADR-0068 mechanism — cert-native anchors, not a synthesis slot

Decision 4 + Status rewritten: band → Table 4b code (C-G) / PCDB index (A/B),
carried by cert-native fields only. Records the review rejection of the
(winter,summer) slot on MainHeatingDetail.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/adr/0068-boiler-efficiency-band-is-the-fifth-heating-companion.md

@@ -2,14 +2,16 @@
 
 ## Status
 
-accepted — implemented & live (band parser + overlay `(winter, summer)` slot +
-calculator precedence + classifier/backfill). C-and-below from accredited Table
-4b; **A/B validated and live for gas/LPG/biogas and oil** — gas from accredited
-Elmhurst RdSAP10 builds (PCDB 18790 → 90.6/81.6, 15029 → 88.0/79.4), oil pinned
-from the accredited PCDB records (17292 → 90.6/82.8, 10498 → 88.0/80.2) after the
-gas builds proved Elmhurst's worksheet (206)/(217) == the PCDB winter/summer
-verbatim (SAP 10.2 App D2.1). Whole-dwelling check: A-rated cert 15017550 +1.78
-SAP; gov-API RdSAP corpus unmoved.
+accepted — implemented & live. Band → **cert-native efficiency anchor** (C-and-
+below: the band's Table 4b `sap_main_heating_code`; A/B: a representative PCDB
+`main_heating_index_number`) — no synthesised field on the cert datatype (see
+Decision 4). **A/B live for gas/LPG/biogas and oil**: gas from accredited Elmhurst
+RdSAP10 builds (PCDB 18790 reg / 15029 combi; 18964 gas-A-combi, 8106 gas-B-reg),
+oil from accredited PCDB (17292, 10498, 15959) after the gas builds proved
+Elmhurst's worksheet (206)/(217) == the PCDB winter/summer verbatim (SAP 10.2 App
+D2.1). Oil A-combi has no >90 % product (≈0 homes) → keeps its code-130 default.
+Each A/B id CI-guarded. Whole-dwelling: A-combi cert 15017550 +2.06 SAP (via PCDB
+idx), D-regular −0.98 (via code); gov-API RdSAP corpus unmoved.
 
 ## Context
 
@@ -68,17 +70,30 @@ gave us), distinct from the sibling **fuel-override data-quality** question
    policy ("we only correct downward") that a later reader would silently "fix"
    into (a) anyway.
 
-4. **Mechanism: `(archetype + fuel) → base code`, then `band → (winter, summer)
-   efficiency slot` that overrides the code's default.** The **archetype and its
-   resolved fuel** pick the *base* `sap_main_heating_code` — this is already
-   ADR-0067's "resolve fuel at resolution" (`Gas boiler, regular` + an oil
-   `main_fuel` → `Oil boiler, regular` → an oil code), and the code still drives
-   system-type behaviour (combi vs regular hot water, etc.). The **band** then
-   sets a `(winter, summer)` seasonal-efficiency pair on the overlay /
-   `MainHeatingDetail` that the calculator consumes **ahead of** the code's
-   Table 4b default — mirroring the existing PCDB precedence
-   (`pcdb_main.winter_efficiency_pct`, `cert_to_inputs.py:2103`). So efficiency is
-   looked up by **(fuel family, boiler type, band)**, never a single annual number.
+4. **Mechanism: `(archetype + fuel) → base code`, then `band → a cert-native
+   efficiency anchor`.** The **archetype and its resolved fuel** pick the *base*
+   `sap_main_heating_code` — ADR-0067's "resolve fuel at resolution" (`Gas boiler,
+   regular` + an oil `main_fuel` → `Oil boiler, regular` → an oil code). The
+   **band** then picks the boiler's efficiency using the **same fields a real
+   cert carries** — deliberately *not* a synthesised efficiency field on the cert
+   datatype (an earlier draft added a `(winter, summer)` slot on
+   `MainHeatingDetail`; rejected in review as it had no lodged-cert analogue and
+   polluted the API-cert type):
+   - **C-and-below** → the band's **Table 4b `sap_main_heating_code`** (regular
+     102→106/101/105/115; combi 104→108/103/107/118). The code is itself
+     combi/regular-specific, so system-type behaviour is preserved by the code
+     alone; the existing Table 4b path resolves the efficiency.
+   - **A/B** (exceed Table 4b's 84 % ceiling, so no code exists) → a
+     **representative PCDB `main_heating_index_number`**, type-matched to the
+     archetype (combi vs regular, because the combi-loss cascade reads the
+     record). `_fold_heating` clears the base code when an index is present, so
+     the **Effective EPC reads as a real PCDB-lodged cert**, resolved by the
+     existing PCDB Appendix D2.1 path (winter + summer + combi-loss). This is also
+     the natural on-ramp to a **future user-supplied product → PCDB id**.
+
+   So efficiency is looked up by **(fuel family, boiler type, band)** and carried
+   by cert-native fields only — never a synthesised number on the cert type. Each
+   A/B product id is CI-guarded to its accredited `(winter, summer)`.
 
 5. **The band mechanism covers exactly the SEDBUK-rated wet boilers — gas and
    oil — and both are currently inflated.** Mains gas, LPG and biogas **share** the