Block bindings, and a field rendered from post meta

A paragraph whose text comes from a meta field had needed a custom block; 6.5 lets a core block bind to a source.

<!-- wp:paragraph {
  "metadata": {
    "bindings": {
      "content": {
        "source": "core/post-meta",
        "args": { "key": "product_sku" }
      }
    }
  }
} -->
<p></p>
<!-- /wp:paragraph -->

The meta key has to be registered with show_in_rest and a sanitisation callback or the binding renders nothing, silently, which is the first thing to check. Custom sources are registrable too and that is where this gets interesting — a binding to an external API is possible and is a rendering-time HTTP call, which is a performance decision disguised as a content one.