fix: a zero-width band fires on equality, not never; dead accessors removed

- The comment in `feed_window()` and three companion sites claimed a zero-width band
  "never fires, because no sample can be on a different side of it than its neighbour".
  That is false: `inside = value >= lo && value <= hi` with `lo == hi == 1.0` is true at
  +1 and false at -1, so a ±1 square fires 19 times in 20 samples.  The correct reason
  the panel refuses one is that it reduces the condition to a float-equality test on a
  real-valued signal, which in practice never fires (fix 4).
- `EventDetector::condition()`, `BufferSearch::channel()` and `BufferSearch::condition_`
  were confirmed unused by grep and deleted (fix 10).

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

src/condition.cpp

@@ -119,8 +119,8 @@ 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 therefore never fires at all, because no sample
-  // can be on a different side of it than its neighbour: the panel refuses one.
+  // 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.
   const bool fired = (cond_.edge == TriggerEdge::Rising && inside) ||
                      (cond_.edge == TriggerEdge::Falling && !inside) ||
                      cond_.edge == TriggerEdge::Both;