perf(ink): replace cache.clear() with LRU eviction in line-width cache

packages/ink/src/line-width-cache.ts

@@ -1,18 +1,35 @@
 import { stringWidth } from './stringWidth.js';
 
-/** Memoised per-line `stringWidth` lookups. */
+/**
+ * Per-line `stringWidth` cache with LRU eviction.
+ *
+ * The renderer calls this ~100k times per frame. Temporal locality is high:
+ * the same lines appear across consecutive frames. An LRU keeps hot entries
+ * resident instead of the old sawtooth pattern (fill → clear-all → cold-start).
+ *
+ * A plain Map provides insertion order, which is sufficient for LRU:
+ * - On hit: delete + re-set promotes the entry.
+ * - On miss with a full cache: evict the oldest key (first in iteration order).
+ */
 const CACHE_LIMIT = 4096;
 
 const cache = new Map<string, number>();
 
 export function lineWidth(line: string): number {
   const hit = cache.get(line);
-  if (hit !== undefined) return hit;
+  if (hit !== undefined) {
+    // Promote: move to the end so it survives the next eviction scan.
+    cache.delete(line);
+    cache.set(line, hit);
+    return hit;
+  }
 
   const width = stringWidth(line);
 
   if (cache.size >= CACHE_LIMIT) {
-    cache.clear();
+    // Evict only the single least-recently-used entry — not the entire cache.
+    const oldest = cache.keys().next().value;
+    if (oldest !== undefined) cache.delete(oldest);
   }
   cache.set(line, width);
   return width;