MiniMax H3 video VAE: decoding a single latent frame (off the 17k+5 grid) produces heavy artifacts — pad-to-2-tokens works; suggest graceful handling or validation

## Summary The MiniMax H3 video VAE **decode** path produces visibly degraded output — 256px tile seams and banding in its default tiled mode, and strong ViT patch-grid artifacts untiled. The **encode** path is fine. ## Repro (master a464ac33, single 1024x1024 image, `minimax_h3_video_vae_fp16.safetensors`) `LoadImage -> VAEEncode -> VAEDecode -> SaveImage`, compare output to input (mean abs pixel error, 0-255): | path | roundtrip error | |---|---| | reference implementation (see below) | **4.6** | | ComfyUI decode, tiled (default `tiling=True`, 256/64) | **31.4** — visible tile seams + banding | | ComfyUI decode, `tiling=False` | **93.7** — 16px patch-grid artifacts | Errors are dtype-independent (fp16 / `--bf16-vae` / `--fp32-vae` within 0.1 of each other), so it's algorithmic, not precision. **Encode is healthy**: comfy-encoded latents match the reference encoder at cosine 0.9997, and decoding a comfy-encoded latent through the reference decoder gives the same 4.6 as a full reference roundtrip. ## Reference implementation The ai-toolkit MiniMax-H3 extension carries an independent port of this VAE ([ostris/ai-toolkit `extensions_built_in/diffusion_models/minimax_h3/src/vae.py`](https://github.com/ostris/ai-toolkit)) that implements the same 256px/64px-overlap spatial windowing with linear blending and reproduces the released pipeline's output (4.6 roundtrip on the same weights file). Diffing the tile layout / per-tile RoPE origin / blend-weight math between the two implementations should localize the defect quickly — the untiled 93.7 result also suggests the ViT decoder is only valid in <=256px windows, so the tiled path is the one worth fixing. Happy to provide the exact test script/images or help bisect. Found while building a custom node that does per-slot decodes of single-frame H3 latents, where the seams are conspicuous on flat backgrounds; the same degradation applies to every normal H3 video decode at >=512px. ## Environment ComfyUI master `a464ac33`, Linux, RTX PRO 6000 (Blackwell), torch 2.14.0.dev+cu132, `--gpu-only`.