Expand fixtures/basic.md beyond prose-only annotations to exercise the
OFM features the parser must model: unordered and ordered lists,
emphasis and strong, a code span, a fenced code block (proving
annotations inside code are ignored), a thematic break, and a
transclusion. Also adds an empty-braces ingredient (@black pepper{}).
Add fixtures/combined.md, a Combined Recipe demonstrating Step
References in both sectioned (#Batter:1) and headingless (#2) forms,
which do not fit cleanly into a standalone recipe file.
