fix: temp_circadian fabricated 0.0 on flat series

lib/src/onehz/foundations/ewma_baselines.dart

@@ -325,6 +325,10 @@ class Baselines {
     final sigma = math.max(1.253 * state.spread, 1e-9);
     final z = (value - state.baseline) / sigma;
     final delta = value - state.baseline;
+    // ponytail: silent 0.0 fallback if baseline is ever exactly 0 — every
+    // metricCfg today has minVal > 0 so update() can never seed a zero
+    // baseline; revisit (make ratio nullable) before adding a metric whose
+    // range legitimately crosses zero.
     final ratio = state.baseline != 0 ? (value / state.baseline - 1.0) : 0.0;
     return Deviation(
         z: z, delta: delta, ratio: ratio, inNormalRange: z.abs() <= 1.0);

lib/src/onehz/wellness/temp_circadian.dart

@@ -356,8 +356,13 @@ CircadianNonparam? _nonparam(
     final d = v - grand;
     varTot += d * d;
   }
+  // A flat/quantized-ADC window has zero total variance and zero
+  // consecutive-difference energy — IS and IV are both undefined divisions
+  // by zero there, not "perfectly stable" 0.0 readings (matches
+  // circadian_np.dart's `if (ssTot == 0) return absent` discipline).
+  if (varTot == 0 || diffN == 0) return null;
   final p = present.length;
-  final iv = (diffN > 0 && varTot > 0) ? (diffSq / diffN) / (varTot / p) : 0.0;
+  final iv = (diffSq / diffN) / (varTot / p);
 
   // IS: between-day stability. Average each within-day epoch-of-day across days,
   // then variance-of-the-24h-profile / total variance.
@@ -393,7 +398,7 @@ CircadianNonparam? _nonparam(
     // circadian_np.dart's `p = epochsPerDay` normalization.
     profVar /= epochsPerDay;
   }
-  final double is_ = varTot > 0 ? (profVar / (varTot / p)).clamp(0, 1) : 0.0;
+  final double is_ = (profVar / (varTot / p)).clamp(0, 1);
 
   // M10 / L5 / RA are DELIBERATELY NOT COMPUTED. See the file header: the temp
   // series that survives retention is median-centred, so it is signed, and

test/onehz/wellness_test.dart

@@ -176,6 +176,23 @@ void main() {
       expect(m.present, isFalse);
       expect(m.confidence, 0);
     });
+
+    test(
+        'nonparam is withheld (not a fabricated 0.0) on a flat/'
+        'zero-variance series', () {
+      // 3 days of an identical ADC reading -> varTot == 0 and diffN's
+      // squared-difference sum == 0. IS/IV are both undefined divisions by
+      // zero here, not "perfectly stable / perfectly regular" measurements.
+      // The cosinor half can still fit a (degenerate) phase on flat input,
+      // so the overall Metric stays present — it's specifically `nonparam`
+      // that must come back null instead of IS=0.0/IV=0.0.
+      final samples = <AdcSample>[
+        for (var i = 0; i < 3 * 24 * 6; i++)
+          AdcSample(i * 10 * 60 * 1000.0, 2000.0),
+      ];
+      final m = tempCircadian(samples, deviceFamily: 'gen4', epochMin: 60);
+      expect(m.value!.nonparam, isNull);
+    });
   });
 
   // -------------------------------------------------------------------------