test: enrich fixtures with full OFM feature coverage

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.

fixtures/combined.md

@@ -0,0 +1,23 @@
+---
+title: Weekend Brunch
+tags: [combined]
+---
+
+# Weekend Brunch
+
+A Combined Recipe that sequences steps from other Recipe Files.
+It has no ingredients or timers of its own — those belong to the referenced recipes.
+
+## Prepare the batter
+
+![[basic#Batter:1]]
+![[basic#Batter:2]]
+
+## Cook and plate
+
+![[basic#Cooking:1]]
+![[basic#Cooking:3]]
+
+## Finish
+
+![[maple syrup#2]]