SAFETY-97: centre the band on the idle reading clamped to 75 W

Owner decision 2026-10-04 (review R1). A wrong idle reading more than
about 100 W off hid every refusal on that battery. Clamped under the band,
the band keeps 0 W +-25 W, so a battery reading about 0 W while refusing is
caught whatever was learned or restored. Samples are still taken to 250 W.
Beyond about +-170 W of idle a battery behaves as in 0.14.0.

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

goodwe_controller/app/idle.py

@@ -9,16 +9,18 @@ Learned per unit, never configured:
 - only while control runs, the unit is not refused, stale or tripped, and
   its command has been within IDLE_ASK_W for IDLE_QUIET_S (a refusing battery
   is asked more than that, so it cannot teach its refusal as idle);
-- a reading beyond IDLE_MAX_W is not a sample, and the value is clamped to it;
+- a reading beyond IDLE_SAMPLE_W is not a sample;
 - first value: the mean of IDLE_FIRST_S of samples; then a slow follow
   (IDLE_TAU_S);
-- persisted per unit and restored, clamped, at start.
-Until learned the idle reading is 0 W, the rule as it was.
+- persisted per unit and restored, clamped to IDLE_SAMPLE_W, at start.
+The band is centred on it clamped to IDLE_MAX_W (owner, 2026-10-04). Until
+learned the idle reading is 0 W, the rule as it was.
 """
 
 import math
 
-IDLE_MAX_W = 250.0      # |idle| bound: a band centred further out hides too much
+IDLE_MAX_W = 75.0       # |idle| used: under the 100 W band, so it keeps 0 W +-25 W
+IDLE_SAMPLE_W = 250.0   # a reading beyond this is not a sample
 IDLE_ASK_W = 50.0       # a command this small counts as "asked ~0"
 IDLE_QUIET_S = 120.0    # 6 lags of 20 s: a 2500 W step has decayed to 6 W
 IDLE_FIRST_S = 120.0    # seconds of samples averaged for the first value
@@ -41,7 +43,7 @@ class IdleReading:
     def __init__(self, saved=None):
         v = _num(saved)
         self.value = (None if v is None
-                      else max(-IDLE_MAX_W, min(IDLE_MAX_W, v)))
+                      else max(-IDLE_SAMPLE_W, min(IDLE_SAMPLE_W, v)))
         self.saved = self.value
         self.quiet_since = None
         self.last = None
@@ -50,7 +52,10 @@ class IdleReading:
 
     @property
     def w(self) -> float:
-        return 0.0 if self.value is None else self.value
+        """The centre used: the learned value clamped to IDLE_MAX_W."""
+        if self.value is None:
+            return 0.0
+        return max(-IDLE_MAX_W, min(IDLE_MAX_W, self.value))
 
     def observe(self, reading, standing_w, now_mono, may_learn) -> bool:
         """One read. Returns True when the value should be persisted."""
@@ -66,7 +71,7 @@ class IdleReading:
             self.quiet_since = now
         b = _num(reading)
         if (now - self.quiet_since < IDLE_QUIET_S or b is None
-                or abs(b) > IDLE_MAX_W or dt <= 0.0):
+                or abs(b) > IDLE_SAMPLE_W or dt <= 0.0):
             return False
         if self.value is None:
             self.acc_w += b * dt
@@ -76,7 +81,6 @@ class IdleReading:
             self.value = self.acc_w / self.acc_s
         else:
             self.value += (b - self.value) * min(1.0, dt / IDLE_TAU_S)
-        self.value = max(-IDLE_MAX_W, min(IDLE_MAX_W, self.value))
         if self.saved is None or abs(self.value - self.saved) >= IDLE_SAVE_W:
             self.saved = self.value
             return True