instrument: fix AHD DecayEnd overlay/grab defect, pin flaky curve test, close comment/doc findings

tests/test_curve_law.cpp

@@ -156,8 +156,11 @@ static void testKnobLawRoundTripsOutsideTheDetent() {
         const double back = curveFromKnobNorm(knobNormFromCurve(e));
         CHECK(std::fabs(back - e) < 1e-9);
     }
-    CHECK(curveFromKnobNorm(0.0) == kCurveMin);
-    CHECK(curveFromKnobNorm(-3.0) == kCurveMin);   // out-of-range norm saturates
+    // exp(-log(10)) is not guaranteed bit-exact to kCurveMin's literal 0.1 (1-2 ulp either way);
+    // both norms below collapse to the same t == 0.0 computation, so both get the same
+    // tolerance rather than leaning on clampCurve's floor to land on it by luck.
+    CHECK(std::fabs(curveFromKnobNorm(0.0) - kCurveMin) < 1e-12);
+    CHECK(std::fabs(curveFromKnobNorm(-3.0) - kCurveMin) < 1e-12);  // out-of-range norm saturates
     CHECK(std::fabs(curveFromKnobNorm(1.0) - kCurveMax) < 1e-12);
     // The ENDS need only land on the domain, not on an exact norm — 0.1 is not exactly 1/10 in
     // binary, so log(kCurveMin) is a hair off -log(kCurveMax). Only the centre carries an