test: add prose-fixture end-to-end smoke for kitchen view (task 0003)

Pull a reduced end-to-end smoke forward into the slice so the view flow
is exercised over a real on-disk recipe, not just a temp file.

Add fixtures/prose.md — a recipe using only this slice's modelled
constructs (frontmatter, headings, plain paragraphs) so it renders
losslessly. integration_test now runs `kitchen view fixtures/prose.md`
as a subprocess and asserts the frontmatter, separator, headings,
paragraphs, and flat document order end-to-end. smoke_test covers
`kitchen --help` exiting 0 with the view command listed.

The full fixtures/basic.md smoke remains task 0007, which needs 0004's
richer nodes before that fixture renders losslessly.

fixtures/prose.md

@@ -0,0 +1,22 @@
+---
+title: Buttered Toast
+servings: 2
+tags: [breakfast, simple]
+---
+
+# Buttered Toast
+
+The simplest breakfast, and a good way to smoke-test the renderer.
+Everything here is plain prose — no lists, code, or annotations yet.
+
+## Method
+
+Toast the bread until golden on both sides.
+
+### Timing
+
+Spread the butter while the toast is still warm so it soaks in.
+
+## Serving
+
+Cut into triangles and serve at once.