fix(metrics): stop rendering stat values and labels as headings

Part of the Markdown for Agents Phase 1 epic (see #5652). kestra.io has Cloudflare Markdown for Agents enabled: `Accept: text/markdown` converts the SSR HTML at the edge, so defects in the HTML end up in what AI agents read. Cloudflare strips `nav`/`header`/`footer`/scripts/styles/inline SVG, ignores `aria-hidden` and empty `alt`, keeps `.visually-hidden` text and `display:none` content, and renders `

` as separate lines (all verified on production).

## Problem
- `src/components/common/Stacked.astro` emits `

6x

` per feature value, giving `## 6x`, `## 90%`, `## 10x` headings on `/`, `/features`, `/use-cases`.
- `src/components/enterprise/StatsHeader.astro` renders the stat label as `

`, giving `##### Faster Workflow Completion` on `/enterprise` (the value is already a `div`).

## Scope
- Stacked: `.cards-row` becomes `

`, each card `

{value}

{label}

`. Scoped CSS mirrors the current h2 metrics: `font-size: $h2-font-size; font-weight: 600; line-height: 1.2; margin-bottom: .5rem`, and `$h3-font-size` below `lg` (see `app.scss` heading rules).
- StatsHeader: `

` label to `

` with `$h5-font-size`, weight 600, `$h6-font-size` below `lg`.

## Done when
- No metric headings in the Markdown of the four pages.
- Visual regression suite passes.

Start with src/components/common/Stacked.astro and src/components/enterprise/StatsHeader.astro, then compare their scoped styles with the heading rules in app.scss. Check the rendered Markdown for /, /features, /use-cases, and /enterprise, and run the visual regression suite; done means no metric headings remain and the suite passes.

Written by the indexing model from the issue text.