feat: a level band and a signal that stops can be the trigger's event too

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

src/condition.hpp

@@ -83,6 +83,8 @@ class EventDetector {
   }
   double feed_edge(double value, double t) noexcept;
   double feed_pulse(double value, double t) noexcept;
+  double feed_window(double value, double t) noexcept;
+  double feed_dropout(double value, double t) noexcept;
   /// Whether a measured width satisfies `cond_.cmp`. `Within`/`Outside` take their two
   /// bounds in either order, so a panel cannot make an empty band by typing the larger
   /// figure into the first spin.
@@ -94,6 +96,9 @@ class EventDetector {
   bool in_pulse_ = false;      ///< PulseWidth: an opening edge has been seen.
   bool pulse_up_ = false;      ///< PulseWidth: its polarity, so we know which edge closes it.
   double pulse_start_s_ = 0.0;
+  bool inside_ = false;        ///< Window: which side of the band the previous sample was.
+  double last_cross_s_ = 0.0;  ///< Dropout: when the signal was last seen crossing.
+  bool dropped_ = false;       ///< Dropout: the timeout has already been reported.
 };
 
 }  // namespace udpscope