fix: deduplicate grid column/shadow width into --nc-column-width

The content-width + 2*spacing formula was repeated independently in
grid-template-columns, body::before shadow, and .bleed-edge. If a
consumer overrode only the grid column, the shadow width diverged.

Introduce --nc-column-width on body as the single source of truth;
shadow and bleed-edge now reference it instead of recalculating.

Author: Leftium

src/_document.scss

@@ -25,10 +25,11 @@
   // On narrow viewports min(..., 100%) caps at viewport width; the real
   // padding-inline on children provides the spacing — no pseudo-padding.
   body {
+    #{$prefix}column-width: #{string.unquote('calc(var(#{$prefix}content-width) + 2 * var(#{$prefix}spacing))')};
     display: grid;
     grid-template-columns:
       1fr
-      #{string.unquote('min(calc(var(#{$prefix}content-width) + 2 * var(#{$prefix}spacing)), 100%)')}
+      #{string.unquote('min(var(#{$prefix}column-width), 100%)')}
       1fr;
     min-height: 100dvh;
     padding-block: var(#{$prefix}spacing);

src/_shadow.scss

@@ -54,8 +54,8 @@
   }
 
   // Content shadow: fixed-position pseudo spanning the full viewport height.
-  // Width matches column 2 (content-width + 2*spacing) — the shadow tightly
-  // wraps the padded column. The real padding-inline on body children creates
+  // Width matches column 2 (--nc-column-width) — the shadow tightly wraps
+  // the padded column. The real padding-inline on body children creates
   // the visual gap between content and shadow edge.
   // The clamp() formula handles visibility: shadow shows when there's at
   // least spacing of clearance on each side between shadow and viewport edge,
@@ -68,8 +68,8 @@
     left: 50%;
     transform: translateX(-50%);
     width: clamp(0px,
-      #{string.unquote('calc((100% - var(#{$prefix}content-width) - 4 * var(#{$prefix}spacing)) * 9999)')},
-      #{string.unquote('calc(var(#{$prefix}content-width) + 2 * var(#{$prefix}spacing))')}
+      #{string.unquote('calc((100% - var(#{$prefix}column-width) - 2 * var(#{$prefix}spacing)) * 9999)')},
+      var(#{$prefix}column-width)
     );
     box-shadow: var(#{$prefix}content-shadow);
     pointer-events: none;
@@ -78,7 +78,7 @@
 
 }
 
-// Break out to shadow/column edge (content-width + 2*spacing).
+// Break out to shadow/column edge (--nc-column-width).
 // Uses the same shadow-visibility condition as the body::before pseudo:
 // when shadow is visible, caps at column width (shadow boundary);
 // when shadow is hidden (narrow viewport), goes full width.
@@ -91,8 +91,8 @@
       grid-column: 1 / -1;
       width: 100%;
       max-width: clamp(
-        #{string.unquote('calc(var(#{$prefix}content-width) + 2 * var(#{$prefix}spacing))')},
-        #{string.unquote('calc((100% - var(#{$prefix}content-width) - 4 * var(#{$prefix}spacing)) * -9999)')},
+        var(#{$prefix}column-width),
+        #{string.unquote('calc((100% - var(#{$prefix}column-width) - 2 * var(#{$prefix}spacing)) * -9999)')},
         100%
       );
       margin-inline: auto;