Release v1.2.1

修复 / Bug Fixes:

• 修复 FPS 计算的两个根本性准确度问题
  • Fixed two fundamental accuracy issues in FPS calculation
  • 问题 1：帧时间戳使用 DateTime.now() 而非帧真实时间戳。由于 addTimingsCallback 是批量回调，同一批多帧会共享几乎相同的时间戳，导致滑动窗口 FPS 计算在窗口边缘抖动、批量到达时失真
  • Issue 1: Frame timestamps used DateTime.now() instead of real frame timestamps. Since addTimingsCallback is batched, multiple frames in the same batch shared nearly identical timestamps, causing sliding-window FPS to jitter at window edges and distort on batch arrivals
  • 修复 1：改用 timing.timestampInMicroseconds(FramePhase.buildStart) 获取每帧真实开始时间戳
  • Fix 1: Use timing.timestampInMicroseconds(FramePhase.buildStart) to get the real per-frame start timestamp
  • 问题 2：帧耗时只算 buildDuration（widget 树构建），漏掉了 rasterizationDuration（GPU 光栅化）。GPU 卡顿是 Flutter 最常见的卡顿类型之一，原实现完全检测不到
  • Issue 2: Frame duration only counted buildDuration (widget tree construction), missing rasterizationDuration (GPU rasterization). GPU jank — one of the most common Flutter jank types — was completely undetected
  • 修复 2：帧耗时改用 rasterFinish - buildStart，包含 build 和 raster 全过程
  • Fix 2: Frame duration now uses rasterFinish - buildStart, covering both build and raster phases
• 修复引入真实时间戳后 FPS 显示 -- 的回归问题
  • Fixed regression where FPS displayed -- after switching to real frame timestamps
  • 根因：_refreshFps() 用 DateTime.now()（wall clock，自 1970 UTC）作为 "now" 来清理旧时间戳，而帧时间戳用的是 FramePhase.buildStart（monotonic time，自引擎启动）。两者时钟基准不同，差值巨大，导致所有帧时间戳在第一次刷新时被全部误清理，FPS 永远为 0
  • Root cause: _refreshFps() used DateTime.now() (wall clock, since 1970 UTC) as "now" to purge old timestamps, but frame timestamps used FramePhase.buildStart (monotonic time, since engine start). Different clock bases caused all timestamps to be erroneously purged on first refresh, making FPS always 0
  • 修复：用帧时间戳中的最大值作为 "now"，确保时钟基准统一
  • Fix: Use the max frame timestamp as "now" to ensure a unified clock base
• 移除未使用的 _frameStartTimes 字段（死代码清理）
  • Removed unused _frameStartTimes field (dead code cleanup)