docs: a zero-width trigger band is refused, not degraded to an edge

The spec claimed a window condition with equal levels degrades to an edge trigger on
that level. It does not: the band test is a change of side, so equal levels make the
signal "inside" only on exact float equality and the condition silently never fires.
Special-casing the detector into an edge trigger would conflate two state machines
that answer different questions -- the divergence the shared evaluator exists to
prevent. The panel refuses the degenerate band instead.

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

docs/superpowers/specs/2026-10-07-scope-features-design.md

@@ -165,8 +165,14 @@ for the same reason every readout in this app quotes a stored sample: t0 has to
 buffer holds, because the cut is a binary search over the time ring and the axis is pinned to
 what the cut got.
 
-A `Window` condition with `level == level2` degrades to an edge on that level, and must — a
-band of zero width is not a special mode.
+**A zero-width band is refused by the panel, not special-cased in the detector.** The band test
+is `v >= lo && v <= hi` and the event is a change of side, so with `level == level2` the signal
+is "inside" only on exact float equality — which a crossing essentially never lands on, making
+the condition one that silently never fires. That is a worse failure than a refusal, and it
+cannot be fixed by special-casing the detector into an edge trigger: the two state machines
+answer different questions and conflating them is the divergence this design exists to prevent.
+So the second level spin is clamped to differ from the first by at least its own increment, the
+panel says why, and `EventDetector` keeps one unconditional band test.
 
 ### Panel and session