perf: move ExtHost initialization to LifecyclePhase.Starting

[j4rviscmd]

Summary

• Move extension host initialization trigger from LifecyclePhase.Ready to LifecyclePhase.Starting in TauriExtensionService
• This allows Bun process spawn, IPC connection, and handshake to overlap with workbench layout creation, reducing perceived startup delay by hundreds of milliseconds
• Update JSDoc comment to reflect the new lifecycle phase

Context

Previously, ExtHost initialization waited until all eager services were instantiated (Ready phase). However, all services needed by the initialization chain (IUserDataInitializationService, workspace context, environment service) are available at Starting phase — and IUserDataInitializationService.initializeInstalledExtensions() is effectively a no-op in the Tauri build (empty initializers array).

The _createExtHostInitData() call (which needs ITelemetryService and other Ready-phase services) runs after Bun spawn + WebSocket handshake completes (~hundreds of ms), by which point Ready has already been reached.

Closes #506

Test plan

• Verify extensions load correctly on startup (e.g., Git extension appears in status bar)
• Verify editors open normally
• Check console logs for extension-related errors
• Regression: if issues arise, reverting to LifecyclePhase.Ready is a one-line change

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