a11y: Fix accessibility violations found by the playwright axe page scan

[coseeian]

This PR clears up the heading-order, and link-name, landmark-unique accessibility violations from a11y page tests:

• Fixed callout heading order: Callout titles were hardcoded as <h5>, breaking document outlines. Converted them to <p class="callout-title"> with role="note" on the container so visual styling stays identical while removing them from the heading structure.
• Fixed empty reference links: Items without a submodule were generating empty focusable links (/reference/#), creating invisible tab stops and triggering link-name failures. We now only render these links when a submodule actually exists.
• Excluded iframe sketch previews from axe scans: srcdoc preview frames generated by p5 were causing duplicate landmark warnings (<main> and canvas regions). Excluded frame contents from the scan scope while keeping iframe elements in scope.

[coseeian]

I'm going to break this up into 3 separate PRs to keep the reviews lightweight and manageable.