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/tools/replay_ac3.py

@@ -176,7 +176,7 @@ def replay(paths, *, guarded: bool, cap_w, max_w, max_hold_s, cycle_s,
             next_cycle = t + timedelta(seconds=cycle_s)
             cycles += 1
             d = decide(PeakTracker.project(ing.averager, t), batt_w=batt,
-                       cap_w=cap_w, reading=tracker.peak(t),
+                       cap_w=cap_w, margin_w=0.0, reading=tracker.peak(t),
                        achievable_w=max_w, max_hold_s=max_hold_s, soc=soc,
                        soc_floor=soc_floor, frozen=False)
             if d.engaged: