Spacing utilities

The spacing utilities are padding and margin one-property helpers across the same xs–xl `--space-*` scale every component's own internal padding and the flex utilities' `.gap-*` already read from. They're meant for arbitrary markup spacing, not for retuning a component's own internal padding — each component already exposes that through its own scoped custom property (e.g. `--btn-pad-x`), overridable directly without a utility class. Because every one of these ultimately reads `var(--space-xs|sm|md|lg|xl)`, overriding the scale once at `:root` retunes every component's internal spacing and every `.p-*`/`.m-*`/`.gap-*` utility on the page at the same time.

<div class="card p-lg mb-md">
  <p class="mt-0">Card content with generous padding and a bottom margin.</p>
</div>