test: add implementation-independent oracle for --json-input --min-ratio

The min-ratio test previously verified `actual` only against a value
produced by replaying the same culling pipeline as the code under test.
Assert directly that an entry far above the threshold survives and one far
below it is culled, so the test no longer depends solely on that pipeline.

https://claude.ai/code/session_01Nj3xEp1eoDKRp1MUCxNQRQ

Author: claude

tests/json.rs

@@ -254,6 +254,12 @@ fn json_input_min_ratio() {
     let unculled = visualize(&apply_pipeline(sample_tree(), 10, 0.0, false));
     eprintln!("UNCULLED:\n{unculled}\n");
     assert_ne!(expected, unculled);
+
+    // Implementation-independent oracle: `foo` is far above the 10% threshold and
+    // must remain, while `bar` is far below it and must be culled. This pins the
+    // culling without reusing the pipeline that produces `expected`.
+    assert!(actual.contains("foo"));
+    assert!(!actual.contains("bar"));
 }
 
 #[test]