feat(flux2klein): Add KV-cache acceleration for FLUX.2-Klein multi-image editing with E2E parity and smoke tests

- Implement prefix extraction phase (step 0) and cached denoising scan loop in FlaxFlux2KleinPipeline
- Support KV cache slicing and concatenation across Flax NNX Double and Single transformer blocks
- Safely bound Splash/Flash attention block sizes for asymmetric cross-attention sequences with opt-in flag
- Add CLI and config support for use_kv with dynamic FLUX.2-klein-9b-kv repository resolution
- Add cross-framework E2E parity test achieving 0.8881 SSIM / 22.58 dB PSNR
- Add 9B KV-cache image editing smoke test with verified golden reference image