warpui_tui: TUI elements (text, column, container, child-view, event-handler)

[zachbai]

Description

Linked Issue

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Testing

• I have manually tested my changes locally with ./script/run

Screenshots / Videos

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• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

[zachbai]

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This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• warpui_tui: crossterm renderer + runtime; wire runtime to TuiPresenter #12417
• warpui_tui: TuiPresenter + first-class TuiBackend::Presenter #12416
• warpui_tui: TUI elements (text, column, container, child-view, event-handler) #12415 👈 (View in Graphite)
• warpui_tui: crate foundation - geometry, buffer, TuiElement, events #12414
• warpui_core: add TuiBackend + TuiView layer behind the tui feature #12413
• warpui_core: parameterize AppContext by a Backend trait (GUI unchanged) #12412
• master

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