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/app/validate.py

@@ -243,6 +243,9 @@ NOT READY banner already names the problem, (b) does not apply.
      note below on `gain`, which is the term that owns that and which SAFETY-39
      measured and did not change.
 
+ 12. `peak_margin_w` outside 0-500 W, not whole, or not below `peak_cap_w`.
+     SAFETY-79; the Supervisor checks the range too, but not for sim/inject.py.
+
 WHAT IS DELIBERATELY *NOT* REFUSED, because it is untidy rather than unsafe:
 
   - `integrator_max_w` above `max_w`. DOCS.md is right that the only thing the
@@ -333,7 +336,7 @@ from .inverter import roster_entries
 from .maintenance import MAINT_OPTION_DEFAULTS
 from .p1 import (
     SOURCE_HA, SOURCE_HA_SIGNED, is_enabled, validate_meter_options)
-from .peakguard import PEAK_OPTION_DEFAULTS
+from .peakguard import PEAK_MARGIN_RANGE_W, PEAK_OPTION_DEFAULTS
 from .price import (
     PRICE_OPTION_DEFAULTS, PROVIDERS, RETIRED_PROVIDERS,
     SUPERSEDED_TAXES_DEFAULTS_EUR_KWH)
@@ -1194,6 +1197,9 @@ def validate_startup_config(opts: dict, dev_min=None, dev_max=None,
                 f"reserve above {bms_floor:.0f} %, or correct the cut-out "
                 "level.")
 
+    # --- 12. SAFETY-79: the margin the peak guard keeps under the limit -----
+    problems += _margin_problems(opts)
+
     # --- warnings: real, but not worth refusing over -------------------------
     integ = opts.get("integrator_max_w")
     integ_w = None if not integ else _number(opts, "integrator_max_w", None)
@@ -2022,6 +2028,23 @@ def _price_check(notes, opts, running, prices_known) -> None:
                 "Prices are fetched after the restart, and Home shows them.")
 
 
+def _margin_problems(opts: dict) -> list[str]:
+    """Rule 12: `peak_margin_w` is a whole number in the schema's range, and
+    below the peak limit, which would leave nothing to aim at."""
+    raw = opts.get("peak_margin_w")
+    margin = _number(opts, "peak_margin_w",
+                     PEAK_OPTION_DEFAULTS["peak_margin_w"])
+    lo, hi = PEAK_MARGIN_RANGE_W
+    if margin is None or margin != int(margin) or not lo <= margin <= hi:
+        return [f"{S('peak_margin_w')} must be a whole number from {lo} to "
+                f"{hi} W. It is {messages.shown(raw)}."]
+    cap = _number(opts, "peak_cap_w", None)
+    if cap is not None and margin >= cap:
+        return [f"{S('peak_margin_w')} is {margin:.0f} W, not below the "
+                f"{S('peak_cap_w')} of {cap:.0f} W. Lower the margin."]
+    return []
+
+
 def _number(opts: dict, key, default):
     raw = opts.get(key, default)
     if raw is None or raw == "":