Variable iframeRuleConst

Block rule for [[iframe URL ...attributes]].

Parsing strategy:

1. Match BLOCK_OPEN + name "iframe".
2. Consume the URL (all tokens until whitespace, BLOCK_CLOSE, or newline).
3. Validate the URL: normalise, reject dangerous schemes, require http(s).
4. Parse key/value attributes, filtering through ALLOWED_IFRAME_ATTRS.
5. Consume closing ]] and optional trailing newline.
6. Emit an iframe element with url and attributes.