Add fade transitions around split-screen wrap on start/stop

[carochacs]

Background

slopsmith-plugin-splitscreen's screen.js fades #splitscreen-wrap in/out around activation and teardown: wrap.style.opacity = '0' at creation, transitioned to '1' once panels are ready in startSplitScreen(), and faded back to '0' (with wrap.style.transition = 'opacity 0.12s ease-out') before teardownPanels() runs in stopSplitScreen() — so entering/exiting split isn't a jarring hard cut.

Current state here

This repo already fades the toast (_showMainToast) and the player HUD, but has no equivalent fade around the #splitscreen-wrap element itself in startSplitScreen/stopSplitScreen.

Ask

Port the wrap opacity fade-in on startSplitScreen() (after panels are created/sized, before/at sizeCanvases()) and fade-out on stopSplitScreen() (before teardownPanels()), matching the existing per-element fade pattern already used for the toast/HUD in this codebase.

Reference

slopsmith-plugin-splitscreen screen.js — wrap.style.opacity usages in createWrap/startSplitScreen and stopSplitScreen.