修正时间轴末帧，导出补阶跃与循环段边界关键帧

- 时间轴：末帧按时长实际覆盖的整数帧计算，修掉 Spine 时长小数舍入多出的一帧，B 端与回放自动停在最后一帧
- 导出：粒子死亡、骨骼池复用、拖尾假死复活处统一写 curve: stepped 硬切，避免死区插值出滑动轨迹
- 导出：循环持续在开始/循环/结束段边界（如 30f/110f）对所有动画通道补关键帧，便于在 Spine 中直接拆分
- 精简器支持阶跃段，阶跃区间内不再补出过渡关键帧
- 补充 test:spine-json 回归用例：骨骼池复用阶跃边界、循环段边界

src/store/particleStore.ts

@@ -5,7 +5,7 @@ import { releaseSpineAsset, retainSpineAsset } from '../spine/spineAssetRegistry
 import { cloneEditorValue } from '../editor/editorClone'
 import { defaultSpineExportSettings, type SpineExportSettings } from '../export/spineExportSettings'
 import { loopSegmentTiming } from '../core/loopSegmentTiming'
-import { normalizePlaybackRange, resizePlaybackRange } from '../timeline/playbackRange'
+import { lastFrameOfDuration, normalizePlaybackRange, resizePlaybackRange } from '../timeline/playbackRange'
 
 export interface ParticleSystem {
   id: number
@@ -519,8 +519,11 @@ export const useParticleStore = defineStore('particle', {
         if (end > maxSec) maxSec = end
       }
       // totalFrames 表示快照数量；0 秒和末端点都可选，因此比末端帧编号多 1。
+      // 末帧编号按“时长覆盖到的最后一个整数帧”计算，不能直接用 ceil：
+      // Spine 的小数舍入会让 80 帧的时长变成 80.01 帧，从而凭空多出第 81 帧，
+      // 回放时表现为末帧之后多停一帧。
       const previousFrames = this.timeline.totalFrames
-      const frames = Math.max(2, Math.ceil(maxSec * fps) + 1)
+      const frames = Math.max(2, lastFrameOfDuration(maxSec, fps) + 1)
       this.timeline.totalFrames = frames
       const range = resizePlaybackRange(
         { start: this.timeline.playbackStart, end: this.timeline.playbackEnd },