[players] Pre-seek next decoder to handle-in to avoid slate flash

[frankrousseau]

Problems

• With handles set on a playlist, continuous playback flashed the next shot's first frame (the slate) at every shot boundary before jumping to the handle-in frame (fixes Handles in playlists not working properly #1019)
• The hidden decoder preloading the next entity stayed parked at t=0; the seek to handle-in only happened inside playNext at switch time, so the newly-current decoder painted frame 0 until that async seek completed

Solutions

• Pre-seek the preloading decoder to the next entity's handle-in time as soon as its source is set (in loadEntity and switchPlayers), so frame 0 is never the frame presented when the players swap
• Feed MultiVideoViewer with a new nextHandleIn prop, computed in PlaylistPlayer from the next entity's own handle data (handles can differ per entity), and re-apply the pre-seek when the prop updates after a switch
• Add a jsdom regression test checking the preloaded decoder is already at the handle-in time after loadEntity and re-parks on handle changes