orca/docs/split-groups-rollout-pr4.md

Split Groups PR 4: Split-Group UI Scaffolding

This branch adds the split-group UI pieces, but does not mount them in the main workspace host yet.

Scope:

• add TabGroupPanel, TabGroupSplitLayout, and useTabGroupController
• add split-group actions to tab menus and tab-bar affordances
• add the follow-up design note for the architecture

What Is Actually Hooked Up In This PR:

• the new split-group components compile and exist in the tree
• tab-bar level split-group affordances are present in the component layer

What Is Not Hooked Up Yet:

• Terminal.tsx does not mount TabGroupSplitLayout in this branch
• users still see the legacy single-surface renderer
• no feature switch exists here because the code path is not wired in yet

Non-goals:

• no rollout to users
• no main-renderer ownership change yet