[Feature] Extract React application environment contract

[TorinAsakura]

Motivation

React support is one of Raijin's original product surfaces, but it is currently mixed with Next-specific behavior and generic renderer helpers. React applications need their own environment contract before Next, React Native, or Astro behavior is layered on top.

Implementation plan

Separate the React application environment from Next-specific behavior. Define which commands, package metadata, runtime assumptions, build outputs, and checks belong to plain React applications. Move shared behavior behind owned capabilities instead of keeping React behavior inside generic renderer utilities.

Definition of done

• Plain React application behavior is separated from Next-specific behavior.
• React build/dev/check command ownership is explicit.
• React environment code does not depend on Next-only assumptions.
• Tests or fixtures cover the React application command surface.
• Existing consumers do not regress during the extraction.