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/help/08-trigger.txt

@@ -34,8 +34,10 @@ share. <b>Arm</b> starts watching; <tt>a</tt> arms and disarms from the keyboard
   behaviour. <b>Pulse width</b> — a crossing and its return, qualified by how long the excursion
   lasted: less than, greater than, within or outside a pair of widths; the event is timed at the
   closing edge. <b>Window</b> — the signal entering or leaving a band between two levels; the
-  panel keeps the two levels at least one increment apart, because a zero-width band reduces the
-  condition to a float-equality test that never fires on a real-valued signal. <b>Dropout</b> —
+  panel keeps the two levels at least one increment apart, because a zero-width band 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.
+  <b>Dropout</b> —
   the absence of a crossing for longer than a timeout; one event per outage, re-arms when a
   crossing returns.
 • The shape's own rows are the only ones shown; the rest are hidden, so the panel never offers