Add drag-to-snap areas for eighths

[devindudeman]

Summary

• Adds compound snap area support for eighths (4x2 grid), following the same pattern used by sixths and fourths
• Dragging to the top or bottom screen edge and sliding along it snaps windows to one of four eighth positions
• Selectable in the Snap Areas preferences tab dropdown
• Eighths appear in their own "Eighths" submenu in the status bar menu
• Particularly useful for ultra-wide monitors

Changes

• New EighthsCompoundCalculation.swift with top/bottom eighths compound calculations
• CompoundSnapArea.swift — added topEighths/bottomEighths cases
• WindowAction.swift — made eighths drag-snappable, added display names, category, and firstInGroup
• WindowActionCategory.swift — added .eighths category
• TerminalCommands.md — updated docs

Test plan

• In Settings > Snap Areas, set the top edge dropdown to "Top eighths from corners; maximize"
• In Settings > Snap Areas, set the bottom edge dropdown to "Bottom eighths from corners; thirds"
• Drag window to top edge, slide left/right — should cycle through 4 top eighth positions
• Drag window to bottom edge, slide left/right — should cycle through 4 bottom eighth positions
• Verify eighths appear in "Eighths" submenu in status bar menu
• Verify existing sixths/thirds/fourths snap areas still work correctly

[rxhanson]

Thanks! I'll test it out soon

[rxhanson]

Here are the assets: eighthsAssets.zip

I would prefer these two actions to be done for this to be merged:

1. Add those assets to the eighths actions
2. Either add shortcut recorders for eighths to the ellipsis popover in the General tab (not the Shortcuts tab), or make it so individual eighths are only added to both the status bar menu and the snap areas by Terminal command. (It's odd to have them selectable in the menus but not configurable).