Supervises the editor runtime: parser, renderer server, and Editor GenServer.

Uses rest_for_one to enforce the dependency chain:

MingaEditor.Supervisor (rest_for_one)
├── Minga.Parser.Manager            Tree-sitter parser Port
├── MingaEditor.Frontend.Manager    Zig/Metal frontend Port
├── MingaEditor.Renderer.Server     Async render pipeline
└── MingaEditor                     Editor orchestration GenServer

If Parser.Manager crashes, everything below restarts. If Frontend.Manager crashes, Renderer.Server and Editor restart. If Renderer.Server crashes, Editor restarts (it holds a resolved pid that would be stale). An Editor crash restarts only the Editor.

This supervisor is conditionally started: it only appears in the supervision tree when the editor UI is active (not in test mode or headless operation).