fix: stack unpositioned subtitle cues so simultaneous subtitles do not overlap

[mvanhorn]

Summary

Simultaneous subtitle cues no longer render on top of each other. External subtitle formats frequently emit multiple active cues with no position (line == Cue.DIMEN_UNSET); the Compose subtitle renderer drew each of them at the same default bottom position, so overlapping dialogue lines stacked onto one another illegibly. A new stackUnpositionedCues() extension assigns unpositioned cues consecutive bottom-anchored line numbers (-1, then upward, accounting for multi-line cue text), which is the same convention ExoPlayer's built-in view uses, and CuesState applies it wherever cues are read.

Why this matters

The reporter in #1698 hit this with subtitles that show two speakers at once — the second cue overprinted the first, making both unreadable. The fix only touches cues with unset positions; explicitly positioned cues pass through untouched, so styled/positioned subtitle formats keep their author-intended layout.

Testing

Added CueExtensionsTest covering: single unpositioned cue gets line -1, two simultaneous cues stack (-1, -2), a multi-line first cue pushes the next cue further up, positioned cues are left unchanged, and empty/singleton lists pass through. The unit test runs under :feature:player:testDefaultDebugUnitTest in CI (no local JVM toolchain on this machine).

Closes #1698