Feature request: support isolated local Codex Desktop dev builds / backend override on macOS

I'm trying to validate Codex fixes in the GUI on macOS without touching my installed Codex.app.

Right now I can build the open-source source tree for the CLI/TUI and app-server path, but I couldn't find a supported way to do either of these cleanly:

1. build a separate local Codex Desktop GUI app from source, or
2. point the installed GUI at a source-built backend/app-server in an isolated, reversible way.

I'd like a documented developer workflow for this.

What would help:

• a supported way to run a separate local Codex Desktop dev instance on macOS
• or a supported backend/app-server override for the GUI
• a way to use separate app data / CODEX_HOME / support directories
• no mutation of /Applications/Codex.app or my default support directories
• explicit rollback / cleanup steps

Context:

• I prototyped the backend side of #21019 in my fork here: https://github.com/vozer/codex/commit/6d76668f032cd9823d0184f32a3a1e2dc4e84ab6
• I'm also leaving a note on #21019 with the prototype link instead of opening an unsolicited PR.

Even if a full local GUI build is not meant to be public, a documented "use the installed GUI with a separate local backend in dev mode" path would already help a lot.