Class DatalayerRuntimeSelector

Selector for Datalayer runtimes and Pyodide. Shows a QuickPick with create commands + controllers.

Architecture Pattern:

• Modeled after vscode-jupyter's LocalPythonKernelSelector
• Shows single QuickPick dialog (not nested menus)
• Mixes command items (Create GPU/CPU) with controller items (Runtimes, Pyodide)
• Command items execute creation flows
• Controller items activate existing controllers