fix(tvos): surf channels the way the list reads

[0xashwin]

Pressing up on the Siri Remote while watching live TV tunes to the channel below the current one, and down tunes to the one above. Every other up/down handler in the app moves the way you'd expect — TVReorderableContentList, the EPG grid — so the player is the odd one out, and it's one left-press away from a channel browser that renders the very same list top-to-bottom.

The original intent was a TV remote's channel rocker, where "up" means a higher channel number. That doesn't survive contact with Lume: the surf list is whatever you launched from — Favorites, Recents, a category — in whatever sort is active. There's no higher channel to go up to. There's just a list on screen.

Why all four engines had it: each host turned the direction into a list offset itself (offset: direction == .up ? 1 : -1), four identical copies, all backwards. So the fix isn't the sign — it's that the hosts were doing arithmetic they had no business doing. Direction now resolves inside LiveChannelNavigator, next to the ordering it indexes into, and the hosts pass .up / .down.

That also makes it testable: MoveCommandDirection doesn't exist on iOS and the test target only runs on iOS sims, which is exactly why 20 existing navigator tests covered offset: 1 and none covered what "up" meant.

Tests: three new cases pinning up → the row above, down → the row below, and up-then-down returning where it started. Confirmed failing before the fix.

[bilipp]

This is actually intended. Would only do this with a setting.