Scaffold linux-gtk: native GTK4/Rust client for traverse-starter

[enricopiovesan]

Summary

Create apps/traverse-starter/linux-gtk/ — a native Linux client using GTK4 and Rust. Phase 1 uses HTTP polling (reqwest) matching web-react. Shares traverse-core-rs crate with the CLI client (see #59). SSE is the phase 2 upgrade path.

Phase 1 scope (HTTP polling — unblocked)

• Native GTK4 app calling the Traverse HTTP/JSON API via reqwest + Tokio
• Text input → POST /v1/workspaces/{workspace}/execute → poll → render result
• Runtime URL + workspace configurable via Preferences dialog
• Health check status in header bar
• GNOME UI conventions: AdwApplicationWindow, AdwHeaderBar, AdwToast

Stack

Concern	Choice
Language	Rust
UI framework	gtk4-rs + libadwaita
HTTP	reqwest (async, tokio runtime)
Architecture	ExecutionState enum in Arc<Mutex<>> — idle/loading/polling/done

Directory structure

apps/traverse-starter/linux-gtk/
  Cargo.toml
  src/
    main.rs
    client.rs                     # reqwest HTTP client (execute + poll)
    execution_state.rs            # state enum + transitions
    ui/
      main_window.rs
      preferences.rs
  tests/
    client_tests.rs

Phase 2 upgrade path (do not implement now)

When Traverse ships #525 / #526 / #527:

• Replace polling with reqwest SSE streaming (byte stream + event parsing)
• Move HTTP + SSE client into shared traverse-core-rs crate (see Extract traverse-core-rs: shared Rust HTTP/SSE crate for linux-gtk and cli-rust #59)
• Replace execute() with send_command("submit", json!({ "note": ... }))

Definition of Done

• cargo build succeeds on Ubuntu 22.04+ with GTK4 + libadwaita installed
• Preferences dialog: manual URL + workspace, persisted to GSettings or config file
• Health check status in header bar
• Note input → execute → poll → renders output fields
• Reset returns to idle
• Unit tests for client.rs
• README in linux-gtk/ covering: prerequisites, system deps, runtime URL config, build and run
• scripts/ci/repository_checks.sh updated to acknowledge linux-gtk/ directory

[enricopiovesan]

Design language reference: docs/design-language.md (added in #74 / #60). Platform client DOD should follow the three-zone IA and field naming there.