fix(danmuku): [PKG-DANMUKU-12] keep frame sampling during async placement

packages/artplayer-plugin-danmuku/src/scheduling-buffer.ts

@@ -0,0 +1,38 @@
+import type { DanmuItem } from './types'
+
+// Sampling owns references only; the public state pools stay under Danmuku's control.
+export default class SchedulingBuffer {
+  private pending = new Set<DanmuItem>()
+  private reserved = new Set<DanmuItem>()
+
+  capture(items: DanmuItem[], failed: Set<DanmuItem>) {
+    for (const item of items) {
+      if (!this.reserved.has(item) && !failed.has(item))
+        this.pending.add(item)
+    }
+  }
+
+  take() {
+    const ready: DanmuItem[] = []
+    const waiting: DanmuItem[] = []
+    for (const item of this.pending) {
+      if (item.$state === 'ready')
+        ready.push(item)
+      else if (item.$state === 'wait')
+        waiting.push(item)
+    }
+    this.pending.clear()
+    const batch = [...ready, ...waiting]
+    this.reserved = new Set(batch)
+    return batch
+  }
+
+  complete() {
+    this.reserved.clear()
+  }
+
+  clear() {
+    this.pending.clear()
+    this.reserved.clear()
+  }
+}