fix(render): keep one frame of PTS headroom

[qiin2333]

改了啥呀

• PTS 调度初始 lead 明确保留一个完整流帧，再叠加 2 ms 的 RenderService 提交余量
• 队列容量从提交余量起算，保留原有突发容纳能力，不让一帧储备偷偷吃掉 burst budget
• 补上 120 FPS 解码抖动、刷新率分档和重锚行为测试

为啥要改

解码后的短时抖动会让帧错过 VSync 槽，表现成偶发粘滞。现在先准备一帧，让这点杂鱼抖动在上屏前被吸收，同时无 PTS 路径完全不动。

验证

• presentation_scheduler_test: passed
• presentation_observability_test: passed
• hvigor assembleApp --mode project -p product=default -p buildMode=debug --no-daemon: BUILD SUCCESSFUL

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📥 Commits

Reviewing files that changed from the base of the PR and between 918b0a5 and 4e9c6b7.

📒 Files selected for processing (3)

• nativelib/src/main/cpp/presentation_scheduler.cpp
• nativelib/src/main/cpp/presentation_scheduler.h
• nativelib/src/test/cpp/presentation_scheduler_test.cpp

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/vsync-slot-scheduler

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