Add collection-based For and ForReturn loop nodes

[JPPhoto]

Summary

This PR adds bounded, collection-based For and ForReturn workflow nodes.

Highlights:

• Adds sequential loop execution with loop-carried LoopState.
• Supports empty collections, early termination, final output aggregation, cleanup, persistence, and resume.
• Supports validated nested For loops and bounded Iterate/Collect bodies.
• Adds explicit collection operations:
  • CollectionConcat
  • CollectionZip
  • CollectionCartesian
• Adds durable direct loop_linkage edges between For and ForReturn.
• Keeps connector paths as editor-only aliases that canonicalize to direct runtime linkage edges.
• Adds backend and frontend graph validation for invalid, duplicate, escaped, cyclic, and mixed loop structures.
• Adds loop output scopes for iteration and final outputs.
• Adds contextual ForReturn discovery and auto-connection in the node picker.
• Adds loop boundary visualization and connector deletion/reconnection handling.
• Regenerates OpenAPI and frontend schemas.
• Adds user and architecture documentation.
• Reuses the existing happy-dom setup without adding browser-test dependencies.

Related Issues / Discussions

QA Instructions

Create some workflows with nested For loops, For loops containing bounded Iterate/Collect bodies. This is what an example For workflow might look like:

Merge Plan

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)