perf: harden chunking against quadratic scans

src/markdown/fences.ts

@@ -73,7 +73,27 @@ export function parseFenceSpans(buffer: string): FenceSpan[] {
 }
 
 export function findFenceSpanAt(spans: FenceSpan[], index: number): FenceSpan | undefined {
-  return spans.find((span) => index > span.start && index < span.end);
+  let low = 0;
+  let high = spans.length - 1;
+
+  while (low <= high) {
+    const mid = Math.floor((low + high) / 2);
+    const span = spans[mid];
+    if (!span) {
+      break;
+    }
+    if (index <= span.start) {
+      high = mid - 1;
+      continue;
+    }
+    if (index >= span.end) {
+      low = mid + 1;
+      continue;
+    }
+    return span;
+  }
+
+  return undefined;
 }
 
 export function isSafeFenceBreak(spans: FenceSpan[], index: number): boolean {