test: add packaged Tauri desktop end-to-end gate

[darwvin-dev]

Scope

• add a test-only e2e Cargo feature for the embedded Tauri WebDriver server
• keep the production Tauri config closed (withGlobalTauri=false) and keep the WebDriver dependency optional
• use a separate E2E config/identifier/capability so test permissions never become the production capability set
• build the actual Tauri debug binary and drive it under Xvfb/DBus with a dependency-free Node W3C WebDriver client
• smoke-test the real desktop window, persisted operations dashboard, universal palette, backend Runbook dry-run, and settings persistence through Tauri IPC/SQLite
• isolate XDG app data inside the CI runner

Safety

The JavaScript E2E side contains no WebdriverIO packages and no execute/mock bridge. e2e/run.mjs uses Node built-in fetch against the loopback W3C WebDriver server. The Rust WebDriver dependency remains optional and registered only behind the e2e feature, so release builds do not expose a WebDriver listener.

npm audit --audit-level=high remains unchanged and passes after pruning the vulnerable WebdriverIO dependency graph.

Stacked PR

This PR is intentionally based on agent/operator-experience-v2 until #31 merges. Retarget to main afterward and require a fresh exact-head CI pass.

Merge gate

No one-shot helper workflow may remain. Merge only after regular Linux/macOS CI, live SSH fixture, strict dependency audit, security gates, and the packaged desktop E2E job all pass on the exact final head.