SAFETY-79: peak guard keeps a margin below the limit

New option peak_margin_w (default 50 W, 0-500 W). The peak guard, the
forecast checks and the maintenance charge sizing aim at
peak_cap_w - peak_margin_w; the month peak, the bill, headroom, warnings
and every notice keep the real cap. Owner decision 2026-09-28.

peak_margin_w 0 is byte-identical to ed739d4 (tools/safety79_identity.py,
test_peak_margin.py part 1). validate.py rule 12 refuses a value outside
0-500, not whole, or not below the peak limit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UBtqPrG1mDLXzTEBnhEkkV

goodwe_controller/DOCS.md

@@ -204,6 +204,7 @@ The label and description each setting shows on the Configuration tab, in tab or
 | Limits | Inverter rating (W) | `max_w` | Your own number: the most power the add-on may ever ask of the inverter. Start low and raise it after commissioning. |
 | Limits | Battery cut-out level (%) | `bms_soc_floor` | The level where the battery's own management system stops giving power. Read it off your battery rather than copying a figure. |
 | Limits | Peak limit (W) | `peak_cap_w` | Your own number: the 15-minute average power the add-on keeps the grid under. Take it from your grid contract. |
+| Limits | Margin below the peak limit (W) | `peak_margin_w` | The add-on aims this much below the peak limit, so a 15-minute period that ends a little high still stays under it. |
 | Limits | 15-minute demand forecast entity (W) | `peak_forecast_entity` | Optional. A forecast of this 15-minute period's average power. Leave it empty and there is no peak limit. |
 | Limits | Peak reserve (%) | `peak_soc_floor` | Peak limiting will not draw the battery below this level, but ordinary use can. Derived value, leave it unchanged. |
 | Tariff | Dynamic price source | `price_provider` | Where prices come from. Choosing a source starts the price fetch, even with trading off. |
@@ -274,6 +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.
 - **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.
@@ -1612,6 +1614,7 @@ help - see "Why the tuning is what it is".
 |---|---|
 | `peak_forecast_entity` | Quarter-hour demand forecast, for capacity-tariff markets. Empty = no cap |
 | `peak_cap_w` | The site's capacity-tariff target |
+| `peak_margin_w` (50) | Watts the peak guard aims under `peak_cap_w` (SAFETY-79). Reports and the bill use `peak_cap_w` itself |
 | `peak_soc_floor` (20) | **The percent below which the capacity-peak guard will not discharge the battery.** ⚠️ It bounds **the peak guard only** — it is **not a pack-wide reserve**, and ordinary self-consumption discharges straight past it (SAFETY-25, see below). Not optional and not the same thing as `maintenance_soc_floor` |
 | `bms_soc_floor` (10) | The pack's **own** cut-out percent, the number the BMS enforces whatever you set above. `peak_soc_floor` must be **strictly above** it or the add-on refuses to start. ⚠️ **At a declared `inverters:` roster this key moves INTO each entry and is REQUIRED there** (SAFETY-32): it describes a battery, and two packs of different models have different cut-outs, so one number for both protects the shallower pack with the deeper pack's BMS. Nothing is inherited — an entry that omits it is refused **by name** at startup, and the flat key below is then not read at all |
 | `price_now_entity`, `price_avg_entity` | Dynamic tariff. Empty = never force a paid grid top-up |