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/ui.cpp

@@ -6983,9 +6983,10 @@ GtkWidget* build_trigger_panel(App& app) {
   app.trigger_level2_spin = spin(-1.0e12, 1.0e12, 0.01, 4);
   gtk_widget_set_tooltip_text(app.trigger_level2_spin,
                               "the band's other edge; it is kept at least one increment away "
-                              "from the level, because a band of zero width reduces the "
-                              "condition to a float-equality test, which on a real-valued "
-                              "signal never fires");
+                              "from the level, because a band of zero width is not a band: the "
+                              "condition becomes 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");
   app.trigger_level2_row = labeled_box("to level", app.trigger_level2_spin);
   gtk_box_append(GTK_BOX(box), app.trigger_level2_row);