解决粒子首位帧衔接问题

src/timeline/playbackRange.ts

@@ -0,0 +1,15 @@
+export type PlaybackRange = { start: number; end: number }
+
+export function normalizePlaybackRange(start: unknown, end: unknown, totalFrames: number): PlaybackRange {
+  const last = Math.max(1, Math.floor(totalFrames) - 1)
+  const safeStart = Math.min(last - 1, Math.max(0, Math.floor(Number(start) || 0)))
+  const safeEnd = Math.min(last, Math.max(safeStart + 1, Math.floor(Number(end) || last)))
+  return { start: safeStart, end: safeEnd }
+}
+
+export function resizePlaybackRange(range: PlaybackRange, previousTotal: number, nextTotal: number) {
+  const previousLast = Math.max(1, previousTotal - 1)
+  const nextLast = Math.max(1, nextTotal - 1)
+  const end = range.end >= previousLast ? nextLast : range.end
+  return normalizePlaybackRange(range.start, end, nextTotal)
+}