guides on the page builder, and negative horizontal on a block

The measure was drawn on Layout only, and this is the screen where things are
placed against it. It also answers the question a negative margin asks: a block
escaping the gutter has to be seen escaping it, against the line it crossed.

Horizontal padding now offers negatives, built from the same scale so there is
still one list.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/sections/BlockSettings.tsx

@@ -134,7 +134,20 @@ export const BlockSettings = ({
                 <option value="">
                   {breakpoint === 'base' ? 'None' : from === null ? 'Inherit (none)' : `Inherit (${from})`}
                 </option>
-                {steps.map((step) => (
+                {/*
+                  Horizontal offers NEGATIVES, vertical does not.
+
+                  A negative horizontal is how a block escapes the gutter its band applied — a video
+                  that runs to the edge of the window inside an inset band. It becomes `-mx-*`, which
+                  cancels the band's `px-*` exactly, so the value to pick is the band's own number.
+
+                  Vertically there is nothing to cancel: a band's vertical padding is the room around
+                  the whole band, and a block pulling itself into the one above is a bug.
+                */}
+                {(axis === 'x'
+                  ? [...steps.filter((step) => step > 0).map((step) => -step).reverse(), ...steps]
+                  : steps
+                ).map((step) => (
                   <option key={step} value={String(step)}>
                     {step}
                   </option>