[MAX] Add Wan I2V diffusion pipeline

## Summary

Add the Wan image-to-video (I2V) diffusion pipeline, extending the T2V pipeline with image conditioning.

## Description

- Extends `WanPipeline` (from modular#6302) with image conditioning support
- Encodes the input image via VAE, zero-pads to full video length, and concatenates with noise latents (36-channel input: 16 noise + 4 mask + 16 condition)
- Compiles a GPU graph for the I2V channel concatenation
- Supports MoE dual-transformer with per-phase LoRA weight swapping
- Input images can be provided as file paths or URLs (downloaded at runtime)
- Architecture registration for `Wan-AI/Wan2.2-I2V-A14B-Diffusers`, `Wan-AI/Wan2.1-I2V-14B-720P-Diffusers`

## Dependencies

Depends on modular#6302 (T2V pipeline) — inherits from `WanPipeline`.

## Checklist

- [x] PR is small and focused
- [x] I ran `./bazelw run format` to format my changes

Assisted-by: Claude Code

Assisted-by: Claude Code

Author: jglee-sqbits