[GATE-P0-2] Add live TUI hands-on target

[sorted-ai-bot]

Summary

• Add traveler_live, a standalone FTXUI ScreenLoop target for CEO hands-on P0-2 PC-1 through PC-4 verification.
• Cover mode switching, leader cheatsheet, callsign autocomplete, ResizableSplit drag-resize, and clean Esc/Ctrl+C exit.
• Wire CI to build the live TUI target without running the interactive binary.

Verification

• xmake build -v traveler_live
• xmake test -v (19/19 targets PASS)

Scope Boundaries

• This PR provides the live hands-on target only.
• This is not a GATE-P0-2 PASS claim.
• PC-11 Core-i3 latency evidence and PC-13 PmoQa formal scoring remain separate blockers.

[github-actions]

PR Auto-Review

PASS: Sanity Check Complete

A - Scope cohesion: PASS
The changes focus exclusively on adding the traveler_live TUI feature for hands-on verification, demonstrating a coherent purpose without unrelated modifications.

B - Commit author: PASS
The author is identified as a human (yumakakuya), not the sorted-ai-bot.

C - Build in Public compliance: PASS
No vendor names or identifiers (Claude/Anthropic/OpenAI) are found in commit messages or code comments.

D - Single-binary discipline: PASS
No runtime dependencies outside of C++ standard libraries and the ftxui package are introduced, maintaining a single-binary discipline.

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• Model: openai/gpt-4o-mini via actions/ai-inference@v2
• Diff: 14007 bytes (truncated=false)
• Run: https://github.com/YumaKakuya/traveler/actions/runs/25709686765

AI-generated sanity check. CEO remains final merge authority.

[sorted-ai-bot]

@YumaKakuya PR #18 is ready for CEO review/merge decision. Scope: P0-2 live TUI hands-on target only; no GATE-P0-2 PASS claim.