feat(flux2): add regional prompting for FLUX.2 Klein

[Pfannkuchensack]

Summary

FLUX.2 Klein's diffusers transformer already accepts an attention mask through joint_attention_kwargs, and its unified attention sequence [txt, img] matches FLUX 1, so regional prompting works without a forward override — we just build a restricted mask and pass it down.

Backend:

• Flux2RegionalPromptingExtension builds a restricted [txt, img] mask from per-conditioning Qwen3 embeddings + image masks (FLUX 1 logic, no CLIP pooled). Unlike FLUX 1 the same mask is applied to every block (no per-block alternation, since we use the diffusers forward).
• flux2 denoise() threads pos_joint_attention_kwargs to all positive transformer forwards; negative passes stay unmasked.
• flux2_denoise invocation accepts list[FluxConditioningField] and skips masking when reference images extend the image stream.

Frontend:

• buildFLUXGraph wires posCondCollect + addRegions for the FLUX.2 path.
• addRegions clones flux2_klein_text_encoder edges (qwen3_encoder, max_seq_len, mask) for each regional prompt.
• validators surface unsupported features (regional negative, auto-neg, reference images) so FLUX.2 regions are filtered consistently with FLUX 1.
• schema.ts regenerated to reflect the list-or-single union.

Related Issues / Discussions

n/a

QA Instructions

1. Load any FLUX.2 Klein model in the canvas.
2. Create two or more Regional Guidance layers with distinct prompts and non-overlapping masks (e.g. left half = "a red car", right half = "a blue truck").
3. Invoke txt2img — generated image should respect the regional split.
4. Verify the unsupported-feature warnings show up on a FLUX.2 region that has:
   • a negative prompt
   • auto-negative enabled
   • a reference image attached
5. Confirm a region with reference images is filtered out (no regional masking applied) — invocation should still run as a global prompt.
6. Sanity-check that a global FLUX.2 Klein generation (no regions) is byte-identical to before this PR (no mask is built when no region masks are present, short-circuit returns None).
7. Regression-check FLUX 1 and Z-Image regional prompting still works.

Merge Plan

Standard merge. No DB/schema migration. schema.ts is regenerated and committed.

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)