Variable footnoteBlockRuleConst

Block rule for [[footnoteblock]].

Parsing strategy:

1. Match BLOCK_OPEN + name "footnoteblock" (case-insensitive).
2. Parse optional attributes (title, hide).
3. Consume closing ]].
4. If ctx.footnoteBlockParsed is already true, fail -- only the first footnoteblock in a document is valid.
5. Set ctx.footnoteBlockParsed = true and emit a footnote-block element.