Fix jitter and flicker in continuous scrolled mode

[lulu2jinchi]

Summary

• stabilize continuous manager location calculation during fast scroll in scrolled-continuous
• • reduce visual flicker around section boundaries on mobile

Root cause

• check() could read stale scroll offsets while prepend/append/recycle was running
• • SCROLLED could fire before check() completed, reporting transient locations
• • visible iframes were repeatedly show()-ed, triggering unnecessary repaint
• • offscreen views were trimmed too aggressively during momentum scroll

Changes

• add getScrollPosition() / syncScrollPosition() and use synced offsets in check()
• • defer SCROLLED emission until queued check() completes; skip stale callbacks via scrolledRequestId
• • call view.show() only when visibility is hidden
• • add scheduleTrim() to delay trim until scroll delta calms down
• • clear trim/scroll timers in destroy()

Scope

• file: src/managers/continuous/index.js
• • no public API changes