Move workspace layout host to AppKit

[lawrencecchen]

Summary

• move the workspace layout shell to the native AppKit host with snapshot-driven tab chrome rendering
• collapse runtime surface identity onto the panel UUID and remove the legacy panelIdFromSurfaceId lookup path from production code
• route split, create, move, reorder, and zoom flows through Workspace.performLayoutCommand and typed panel helpers
• isolate browser mounting and portal assumptions behind a dedicated workspace content adapter
• document the resulting architecture and migration end state in docs/workspace-layout-simplification-plan.md

Testing

• ./scripts/reload.sh --tag ws-layout-simplify
• xcodebuild -project GhosttyTabs.xcodeproj -scheme cmux-unit -configuration Debug -clonedSourcePackagesDirPath "$PWD/.ci-source-packages" -disableAutomaticPackageResolution -destination 'platform=macOS' test

Issues

• Closes Rip out Bonsplit and replace it with a direct AppKit split host #2289