chore: flatten src/ to root

ink/line-width-cache.ts

@@ -0,0 +1,24 @@
+import { stringWidth } from './stringWidth.js'
+
+// During streaming, text grows but completed lines are immutable.
+// Caching stringWidth per-line avoids re-measuring hundreds of
+// unchanged lines on every token (~50x reduction in stringWidth calls).
+const cache = new Map<string, number>()
+
+const MAX_CACHE_SIZE = 4096
+
+export function lineWidth(line: string): number {
+  const cached = cache.get(line)
+  if (cached !== undefined) return cached
+
+  const width = stringWidth(line)
+
+  // Evict when cache grows too large (e.g. after many different responses).
+  // Simple full-clear is fine — the cache repopulates in one frame.
+  if (cache.size >= MAX_CACHE_SIZE) {
+    cache.clear()
+  }
+
+  cache.set(line, width)
+  return width
+}