Improve multi-target watch UX for client/server/RSC bundle workflows

[justin808]

Summary

For React on Rails RSC setups, local development often requires multiple concurrent bundle processes:

• client dev server
• server bundle watch
• RSC bundle watch

This is workable but creates operational overhead and confusion around which process owns which output.

Request

Improve multi-config watch ergonomics in Shakapacker so users can run fewer commands for common split-bundle flows.

Suggested Options

1. Add a first-class multi-target watch mode (e.g., --watch-targets=client,server,rsc).
2. Provide an official documented recipe/helper for multi-config watch orchestration with clear defaults.
3. Expose machine-readable status output so wrapper tools can supervise all bundle targets consistently.

Acceptance Criteria

• A standard command path exists for watching multiple bundle targets used in server-rendered/RSC apps.
• Documentation includes expected process model and troubleshooting guidance.

Duplicate Check

I searched for existing issues on this exact multi-target watch UX and did not find a duplicate.

Related

• Tracking issue: Tracking: Improve gem/npm version drift DX and verification tooling #933