warpui_tui: crate foundation - geometry, buffer, TuiElement, events

[zachbai]

Description

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• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

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

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[zachbai]

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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
• warpui_tui: crate foundation - geometry, buffer, TuiElement, events #12414 👈 (View in Graphite)
• warpui_core: add TuiBackend + TuiView layer behind the tui feature #12413 : 1 other dependent PR (#12601 )
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[zachbai]

Superseded: the warpui_tui crate was absorbed into warpui_core behind a purely additive tui feature. The content of this PR (ported and adapted to the unified View API with a typed render path) now lands via #12601, on the consolidated foundation #12413.