fix(builder): sanitize CSS-value sinks to prevent style/<style> breakout XSS

Adds a single cssValue() sanitizer (src/utils/escape.ts) that strips
<>{};"'\ and neutralizes url(), safe for both style="..." attribute and
<style>...</style> element contexts. Applies it at every raw user-prop
CSS-value interpolation sink found via grep across src/components (colors,
sizes, gaps interpolated directly into style strings/<style> blocks),
including the highest-risk <style>-context sinks: ColumnLayout gap,
Menu/Navbar hover and background colors. Also Number()-coerces the
`columns` grid-template-columns sinks in Gallery/Testimonials/NumberCounter
as defense in depth. Regression tests assert </style><script> payloads are
neutralized and normal colors still render.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/components/layout/ColumnLayout.tsx

@@ -2,7 +2,7 @@ import React, { CSSProperties } from 'react';
 import { useNode, Element, UserComponent } from '@craftjs/core';
 import { Container } from './Container';
 import { cssPropsToString } from '../../utils/style-helpers';
-import { escapeAttr, scopeId } from '../../utils/escape';
+import { escapeAttr, scopeId, cssValue } from '../../utils/escape';
 
 type SplitOption =
   | '100'
@@ -117,7 +117,11 @@ ColumnLayout.craft = {
 (ColumnLayout as any).toHtml = (props: ColumnLayoutProps, childrenHtml: string, nodeId?: string) => {
   const columns = props.columns || 2;
   const split = props.split || '50-50';
-  const gap = props.gap || '16px';
+  // Sanitized once here so BOTH the raw <style> nth-child rule below AND the
+  // cssPropsToString-built outerStyle get a safe value -- gap is a raw
+  // string-interpolation sink into a <style> block (worst case: </style>
+  // breakout -> arbitrary <script>), see task-cssxss-brief.md.
+  const gap = cssValue(props.gap) || '16px';
   const widths = getWidths(split, columns);
 
   const outerStyle = cssPropsToString({