test(pipeline): the page-swap test's positive control asserts the source ref (#157)

Review round 2 on PR #176: the control checked for text that was in the
page before BackfillRefs ran, so the test stayed green when no page was
read at all. It now asserts the [[s|S]] ref BackfillRefs adds; red when
the page read always fails, still red when the read follows the link.
Branch rebased onto main.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/pipeline/root_test.go

@@ -162,6 +162,6 @@ func TestBackfillRefsDoesNotCopyAPageSwappedInBehindASymlink(t *testing.T) {
 		return string(b)
 	}
 	require.NoError(t, os.WriteFile(page, []byte(inside), 0o644))
-	require.Contains(t, read(), "zqinside", "positive control: the regular page gets the ref")
+	require.Contains(t, read(), "[[s|S]]", "positive control: the regular page gets the source ref")
 	brainfstest.AssertNoLeakFrom(t, page, inside, target, 3*time.Second, read)
 }