#2504: the dehaze regression test also fails when the whole dark field renders black

Review follow-up (CodeRabbit): an all-black field made every comparison pass, so the field's own level must stay visible first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AbegtYaPuwqZ81pGfELsFK

crates/pipeline/src/tests.rs

@@ -96,6 +96,8 @@ fn dehaze_does_not_crush_dark_regions_next_to_bright_ones() {
         }
         v.sort_unstable();
         let mid = v[v.len() / 2] as i32;
+        // a field rendered all black would pass the comparison below vacuously
+        assert!(mid > 2, "the dark field itself is crushed in ch {ch}: level {mid}");
         for y in 0..160 {
             for x in 0..120 {
                 let p = r.get(x, y)[ch] as i32;