SAFETY-79 review: margin under a banked month peak too; identity harness reaches headroom and maintenance

- quarter_target_w() is now the margin under quarter_ceiling_w(): a peak
  banked between cap - margin and the cap was aimed at exactly, so one
  overshoot crossed the cap; above the cap the same overshoot raised the bill.
- safety79_identity.py hashes charge_headroom_w() and the shaping suspension,
  and adds n1_maint/n2_maint (cheap window, forecast across cap - 50 .. cap).
  Digests re-printed on ed739d4 and pinned; margin 0 still identical.
- test_margin_under_banked_peak; DECISIONS and DOCS updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

goodwe_controller/DOCS.md

@@ -275,7 +275,7 @@ When the add-on refuses to start, each reason names the setting by its label and
 - **Inverter rating** (`max_w`): your own number, not a derived one. It is not read when the inverter list is filled in, since each entry carries its own and the site limit is their sum. Several safety thresholds must stay below it, and the add-on refuses to start if one does not.
 - **Battery cut-out level** (`bms_soc_floor`, `inverters.bms_soc_floor`): measured, not chosen. Register 0x0560 reads 10 % on the reference GW5000S-BP. It exists so the add-on can refuse a “Peak reserve” at or below it, which could never take effect. Each inverter entry states its own, because one default for two different batteries protects the shallower one with the deeper one's number.
 - **Peak limit** (`peak_cap_w`): your own number, from your grid contract. In Belgium it is the capacity-tariff peak your distribution charge is billed on.
-- **Margin below the peak limit** (`peak_margin_w`): 50 W by default, 0 to 500 W (SAFETY-79). Peak limiting aims at the peak limit minus this margin, because a 15-minute period aimed exactly at the limit can end a watt or two over it (3501.9 W against 3500 W in a test), and even 1 W over sets the billed peak. The bill, this month's peak and the warnings still compare against the peak limit itself. 0 aims at the limit, as before.
+- **Margin below the peak limit** (`peak_margin_w`): 50 W by default, 0 to 500 W (SAFETY-79). Peak limiting aims at the peak limit minus this margin, because a 15-minute period aimed exactly at the limit can end a watt or two over it (3501.9 W against 3500 W in a test), and even 1 W over sets the billed peak. Once this month's peak is already above the limit, it aims the same margin below that peak. The bill, this month's peak and the warnings still compare against the peak limit itself. 0 aims at the limit, as before.
 - **Peak reserve** (`peak_soc_floor`): derived, not picked (SAFETY-24). The reference battery holds 13.8 kWh usable and the worst modelled peak day asks for 878 Wh, or 6.4 %. 20 % sits ten points above the cut-out level and leaves about 1.6 worst days of room. It bounds peak limiting only: ordinary use goes on down to the cut-out level. It must stay above “Battery cut-out level”.
 - **Dynamic price source** (`price_provider`): selecting a provider is what starts the fetch, every half hour, whether or not trading is on. It may not be combined with the two older price entities. engie_dynamic, totalenergies_dynamic and eneco_eddy are withdrawn and refuse to start: their endpoints never existed. They stay in the list only so such a site gets that explanation.
 - **Supplier markup** (`price_markup_eur_kwh`): under manual_csv, whose prices are all in, it is ignored and logged as ignored.