[Feature]: Add shortcut for Move Tab to Split Down

Problem or use case

I want to move the active tab into a new Orca pane below using only the keyboard. Today this is available from the tab context menu under Move Tab to Split → Down, but the equivalent action is not exposed in Settings → Shortcuts.

The shortcut settings do expose Split terminal down, but that creates an internal split inside a terminal tab. It does not create an Orca-level tab/pane split. My common workflow is to keep an agent or editor above and move a terminal tab into an Orca pane below, and having to use the mouse and submenu every time adds friction.

Proposed solution

Add a tabs-scoped, bindable action such as tab.moveToSplitDown under Settings → Shortcuts.

It should perform exactly the same operation as Move Tab to Split → Down in the tab context menu: move the active tab into a new pane group below, for any active tab type. It would be fine to ship unbound so users can choose their own shortcut.

Ideally, expose direction-specific actions for all existing menu directions: Right, Left, Down, and Up. The main need here is Down.

Alternatives or additional context

Related: #10055 and PRs #10067 / #10076 add Move Tab to Split Right only; that issue explicitly leaves Down, Left, and Up menu-only. #16464 / #16465 cover moving to a side-by-side pane column, not moving the tab into a pane below.

The screenshot below shows the existing menu action that needs a bindable shortcut.

Start in the existing Settings → Shortcuts entry for Split terminal down and trace the Move Tab to Split → Right action referenced by #10055 and PRs #10067 / #10076. Add a bindable tabs-scoped action for moving the active tab into a pane below, matching the context-menu behavior for every active tab type; verify that it can be assigned in Settings → Shortcuts and performs the Down operation.