fix: a zero-width band fires on equality or not at all, and the text now says which

The earlier correction moved the claim rather than fixing it: four places still
said the float-equality test "never fires on a real-valued signal", and a +/-1
square is a real-valued signal -- a standalone check against a band of [1, 1]
fired 19 times, because `inside` is true at +1 and false at -1, so the signal
changes side on every transition.

All four now state the measured behaviour: an exact float-equality test against
the level, which an ordinary signal never satisfies and a signal sitting at the
level satisfies on every transition. Neither answer is a band, which is the
reason the second spin is clamped an increment away rather than the detector
special-cased -- the same conclusion, now resting on a true premise.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/condition.cpp

@@ -119,8 +119,11 @@ double EventDetector::feed_window(double value, double t) noexcept {
   if (inside == inside_) return kNoEvent;
   inside_ = inside;
   // The event is the *change of side*, so `Rising` reads as "into the band" and `Falling`
-  // as "out of it". A band of zero width reduces the condition to an equality test on a
-  // real-valued signal, which is a float-equality test and therefore unusable: the panel refuses one.
+  // as "out of it". A band of zero width is not a band: `inside` reduces to an exact
+  // float-equality test against the level, so the condition fires on none of an ordinary
+  // signal's samples and on every transition of one that sits exactly at the level -- a
+  // +/-1 square against a band of [1, 1] fired 19 times in a standalone check. Neither
+  // answer is a band, so the panel refuses one.
   const bool fired = (cond_.edge == TriggerEdge::Rising && inside) ||
                      (cond_.edge == TriggerEdge::Falling && !inside) ||
                      cond_.edge == TriggerEdge::Both;