Graph the artwork thrown out, not the artwork kept (#198)

The sweep trims the cache to its limit, so the kept bytes it reports are the
limit every time once the cache has filled: the panel was two lines flat at
500 MiB. A counter of images thrown out says whether the limit is too small.

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

src/art.rs

@@ -98,6 +98,7 @@ pub fn trim(dir: &Path, limit: u64) -> usize {
         }
     }
     crate::metrics::STATS.art_cache_bytes.store(total, std::sync::atomic::Ordering::Relaxed);
+    crate::metrics::STATS.art_trimmed.fetch_add(gone as u64, std::sync::atomic::Ordering::Relaxed);
     gone
 }