warpui_core(tui): add ratatui-backed TUI presenter and runtime

[kevinyang372]

Description

Add the ratatui-backed TUI presenter and runtime on top of the element layer, completing the in-core TUI rendering backend behind the tui feature. Stacked on #12758.

What changed:

• presenter::tui::TuiPresenter — immediate-mode measure → arrange → present → paint into a ratatui Buffer, reporting child-view embeddings into the shared neutral view hierarchy.
• runtime::TuiRuntime — drives a TuiView window: raw-mode + alternate-screen RAII guard, invalidate → redraw, crossterm input → shared keymap then element-tree dispatch, and draining of deferred app updates / typed actions.
• TuiFrameRenderer flushes via ratatui's Buffer::diff + CrosstermBackend (no hand-rolled diff); crossterm comes from ratatui::crossterm.
• event_conversion maps crossterm 0.29 input to the shared Event vocabulary (identical dispatch to the GUI).
• Re-export Color / Modifier from elements::tui so consumers can style text without depending on ratatui directly.
• Flicker fix: a resize repaints authoritatively (clear + redraw) wrapped in a terminal synchronized update, so the frame is applied atomically — no blank flash and no stale fragments left from the previously-wrapped frame.
• Added an interactive example (examples/tui_demo.rs) for manual validation.

Linked Issue

No linked issue — internal, WIP infrastructure gated behind the tui feature (not user-visible).

Testing

• cargo nextest run -p warpui_core --features tui — 360 pass, including the end-to-end tui_integration test (model → view → presenter → typed-action dispatch) plus renderer / runtime / event-conversion unit tests.
• cargo test -p warpui_core --features tui --doc — pass; default build and clippy (both default and --features tui) clean; ./script/format clean.
• Manual: cargo run -p warpui_core --example tui_demo --features tui — verified paragraph wrapping (resize to re-wrap), emoji / CJK / ZWJ column alignment, the buffer diff, vertical scrolling, and flicker-free resizing.

Screenshots / Videos

Manual TUI smoke test is via the example above; ./script/run is not applicable here (it launches the GUI app, not the TUI backend).

Agent Mode

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

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Plan: https://staging.warp.dev/drive/notebook/RQNlhRWhQK3Djnr50YbC4G
Conversation: https://staging.warp.dev/conversation/9293e035-3939-42c7-aeee-110a5a131011

CHANGELOG-NONE

[kevinyang372]

Warning

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.
Learn more

• get tool calls working #12849
• string together input editor + input / output + scrolling #12827
• add tech spec #12794
• Prototype TUI #12786
• warpui_core(tui): add ratatui-backed TUI presenter and runtime #12759 👈 (View in Graphite)
• warpui_core(tui): build TUI element primitives on ratatui-core #12758
• warpui_core: TUI element library (text, column, container, child_view, event_handler) #12634 : 1 other dependent PR (#12601 )
• Add TUI API surface to AppContext under tui #12633
• warpui_core: backend-neutral view hierarchy for a shared responder chain #12413 : 1 other dependent PR (#12414 )
• master

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