Feature: flux2 klein lora support

[Pfannkuchensack]

Summary

Add full LoRA support for FLUX.2 Klein models (4B and 9B variants), including automatic detection, loading, key conversion, and UI integration.

Model Detection & Configuration

• Detect FLUX.2 Klein LoRAs during model probe by inspecting tensor shapes (context_embedder, vector_in, hidden size, MLP ratio) to distinguish them from FLUX.1 LoRAs
• Auto-detect Klein variant (4B vs 9B) from tensor dimensions and store as variant field on the config
• New LoRA_LyCORIS_Flux2_Config and LoRA_Diffusers_Flux2_Config model configs
• Register FluxLoRAFormat.BflPeft as a new recognized LoRA format
• Fix Config_Base.get_tag() to handle None defaults (needed for the optional variant field)

BFL PEFT LoRA Format Support

• New flux_bfl_peft_lora_conversion_utils.py with full BFL PEFT format detection and conversion
• Handles fused QKV splitting (BFL fuses Q/K/V, diffusers uses separate projections)
• Two conversion paths: FLUX.1 (keeps BFL keys) and FLUX.2 Klein (converts to diffusers keys)
• Runtime fallback conversion via convert_bfl_lora_patch_to_diffusers() for LoRAs loaded with FLUX.1 format but applied to FLUX.2 models

LoRA Loading & Application

• New Flux2KleinLoRALoaderInvocation and Flux2KleinLoRACollectionLoader invocations for applying LoRAs to Klein's transformer and Qwen3 text encoder
• Variant mismatch warnings when a LoRA's variant doesn't match the transformer

Frontend

• New addFlux2KleinLoRAs.ts graph builder for wiring LoRAs into the Klein generation graph
• LoRA picker filters by variant compatibility (only shows 4B LoRAs for 4B models, etc.)

Related Issues / Discussions

QA Instructions

1. Install a FLUX.2 Klein model (4B or 9B)
2. Install a LoRA trained for that Klein variant (in BFL PEFT, diffusers, or LyCORIS format)
3. Verify the LoRA is auto-detected as flux2 base with the correct variant (4B/9B) in the Model Manager
4. Verify the LoRA picker only shows variant-compatible LoRAs when a Klein model is selected
5. Generate an image with the LoRA applied and verify it takes effect without shape errors
6. If possible, test with a mismatched variant LoRA and verify a warning is logged

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)

[Pfannkuchensack]

I added the variant Fields for Loras too and maybe we could use that for SDXL Subtypes too.