Add detectEdgeSectionPadding (12th pre-validation detector). Flags a
mobile-shaped page root (width 320–480) when its horizontal padding is
0 AND a child content section also has 0 left padding AND that section
contains visible text or icon descendants — the chain that produced the
"Categories" no-padding bug. Suggested fix sets root.padding to
[top, 16, bottom, 16] preserving vertical padding.
False-positive guards skip top-nav / bottom-nav / hero / banner roles
and image-only sections that are intentionally full-bleed.
The new detector lives in its own detectors-spacing.ts since detectors.ts
is already over the 800-line file limit.
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