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/app/peakguard.py

@@ -143,9 +143,9 @@ because the money is spent on `max(month's peak so far, this quarter)`:
 
     ceiling_w = max(peak_cap_w, the month's banked peak)
 
-SAFETY-79: the guard AIMS at `max(peak_cap_w - peak_margin_w, banked peak)`
+SAFETY-79: the guard AIMS `peak_margin_w` under that ceiling
 (`quarter_target_w()`), so a quarter that lands a little high still stays under
-the cap. `ceiling_w` stays the real limit for reports and the bill.
+it. `ceiling_w` stays the real limit for reports and the bill.
 
 Once a month's peak already stands at 5013 W, holding every later quarter to
 3500 W buys nothing - the bill for that month is already 5013 W - while the
@@ -536,10 +536,12 @@ def quarter_ceiling_w(cap_w, reading) -> tuple[float, str]:
 
 
 def quarter_target_w(cap_w, margin_w, reading) -> float:
-    """SAFETY-79. The average the guard AIMS at: `quarter_ceiling_w()` with the
-    cap lowered by `margin_w`. A month peak already paid for is aimed at as it
-    stands. Reports and the bill keep the real cap (`ceiling_w`)."""
-    return quarter_ceiling_w(float(cap_w) - float(margin_w), reading)[0]
+    """SAFETY-79. The average the guard AIMS at: `margin_w` under
+    `quarter_ceiling_w()`, so under a month peak already banked as well as
+    under the cap. Aiming AT a banked peak can bank a few W over it, which
+    raises the bill as surely as going over the cap. Reports and the bill keep
+    the real limit (`ceiling_w`)."""
+    return quarter_ceiling_w(cap_w, reading)[0] - float(margin_w)
 
 
 def excess_w(projection, ceiling_w) -> float: