Add nested list rendering to formal mode

[p13i]

Gap

_gather_list treats an indented line as continuation text,
so nested ordered and unordered lists are flattened into one
list item. The formal renderer documents a Markdown subset
and does not currently claim nested-list support.

Acceptance criteria

• Add a neutral fixture containing an unordered list nested
  in an ordered list and the reverse.
• Render semantic nested <ul> and <ol> elements.
• Preserve the current behavior for flat lists and wrapped
  list-item text.
• State the added subset behavior in the public docs.

Verification

make check

Full CommonMark list compatibility is outside this issue.

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Written by Codex