Fix(frontend): Make ordering of multiple FLUX.2 reference images deterministic

[lstein]

Summary

When building the FLUX.2 invocation graph, reference images were fed into a single collect node via multiple item edges. Because node execution order isn't guaranteed, the resulting collection order was non-deterministic — causing inconsistent behavior in Flux2RefImageExtension (which uses index-based T-coordinate offsets per image).

Fix: Replace the single collect node with a chain of collect nodes, one per reference image. Each node accumulates the previous collection and appends the next item, guaranteeing insertion order matches the source array order.

Before:

const flux2KontextCollect = g.addNode({ type: 'collect', ... });
for (const { config } of validFlux2RefImageConfigs) {
  const kontextConditioning = g.addNode({ type: 'flux_kontext', ... });
  g.addEdge(kontextConditioning, 'kontext_cond', flux2KontextCollect, 'item');
}
g.addEdge(flux2KontextCollect, 'collection', flux2Denoise, 'kontext_conditioning');

After:

let prevCollect: Invocation<'collect'> | null = null;
for (const { config } of validFlux2RefImageConfigs) {
  const kontextConditioning = g.addNode({ type: 'flux_kontext', ... });
  const collectNode = g.addNode({ type: 'collect', ... });
  g.addEdge(kontextConditioning, 'kontext_cond', collectNode, 'item');
  if (prevCollect !== null) {
    g.addEdge(prevCollect, 'collection', collectNode, 'collection');
  }
  prevCollect = collectNode;
}
assert(prevCollect !== null);
g.addEdge(prevCollect, 'collection', flux2Denoise, 'kontext_conditioning');

Related Issues / Discussions

closes #8923 (superseded)

QA Instructions

With multiple FLUX.2 reference images, verify that generation is stable across repeated runs (same images, same seed → same output).

Merge Plan

Simple merge

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)