add reusable TUI tab bar component

[harryalbert]

Description

Adds TuiTabBar, a retained horizontal tab component for the shared TUI element library. Callers own semantic state—the ordered tabs, selected key, focus, and page anchor—while the component owns stable pointer state, display-cell-aware packing, overflow boundaries, and settled keyboard navigation. It supports an optional fixed main tab, pageable secondary tabs, caller-rendered leading elements, focused/unfocused selection styles, grapheme-safe truncation, semantic click events, and built-in tab/arrow hover affordances.

The main implementation is crates/warpui_core/src/elements/tui/tab_bar.rs. The module documentation at the top of that file is the best entry point: it explains ownership, the per-frame layout flow, how leading elements are measured and reused, how pure paging feeds the rendered row, and why only settled navigation is retained after layout.

Shared terminal-string measurement and grapheme-safe ellipsis logic lives in text_helpers.rs and is reused by both the tab bar and existing TUI column formatting. The tests are intentionally scenario-based rather than one test per field/state.

This PR only introduces the reusable primitive. No user-visible surface consumes it yet; the orchestration integration is in the PR above, so screenshots/video are not applicable here.

Linked Issue

CODE-1822 — Orchestration

Review guide

• crates/warpui_core/src/elements/tui/tab_bar.rs — main component, retained-state boundary, row construction, paging, navigation, pointer interaction, and top-level architecture documentation.
• crates/warpui_core/src/elements/tui/text_helpers.rs — shared display-cell width and grapheme-safe truncation.

Testing

• I have manually tested my changes locally with ./script/run-tui
  No visual changes yet, that'll come in the PR above this one in the stack

Agent Mode

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

[harryalbert]

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

• add product and tech spec #13855
• Add TUI orchestration tab bar specs #13832
• add reusable TUI tab bar component #13831 👈 (View in Graphite)
• rich message component #13777
• Add local child agents to TUI #13776
• add TUI multi-session registry and focused-session root projection #13775
• add TUI orchestration permission and configuration card #13717
• add reusable TUI option selector over shared option snapshots #13716 : 1 other dependent PR (#13830 )
• add reusable TUI editor view #13817
• route TUI editor input by owning-view focus #13771
• add option snapshots and adapt GUI orchestration pickers onto them #13714
• extract orchestration edit state into a frontend-neutral module #13707
• master

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[oz-for-oss]

@harryalbert

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[oz-for-oss]

Overview

This PR adds a retained, reusable TUI tab bar component plus shared text-width/truncation helpers and scenario-based tests. The implementation generally follows the intended retained-state and paging model, and I did not find security issues in the changed code.

Concerns

• Keyboard navigation falls through to the off-page fallback even when there is no valid selected key, so callers can unexpectedly select a visible tab from an unselected or stale-selection state.

Verdict

Found: 0 critical, 1 important, 0 suggestions

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[oz-for-oss]

⚠️ [IMPORTANT] selected_is_visible == false also covers selected_key == None or a stale key, so this fallback makes keyboard navigation select the first/last visible tab when there is no current selection. Return None unless the selected key exists in the supplied order, then use the off-page fallback.

Suggested change

	match direction {
	selected_index?;
	match direction {

[kevinyang372]

My feedback is instead of building a dedicated tab bar element, we should have a tab bar view that renders with generic elements

This should be do-able given we have sizing-aware conditional elements implemented in GUI -- we could probably reference that and built it for TUI