[MAX] Add UniPC multistep scheduler for Wan diffusion

## Summary

Add a numpy-only UniPC multistep scheduler for Wan diffusion pipelines.

## Description

- Implements the UniPC-BH2 algorithm with corrector and predictor steps
- Supports flow-matching sigma schedules (used by Wan 2.1/2.2)
- Provides `build_step_coefficients()` to precompute per-step coefficient matrices on the host, enabling on-device scheduler steps without Python-side numpy calls during denoising
- Registers `UniPCMultistepScheduler` in the diffusion scheduler factory

This is a numpy-only port of the diffusers `UniPCMultistepScheduler`, specialized for the Wan pipeline configuration.

## Dependencies

None — can be merged independently.

## 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

stack-info: PR: #13, branch: jglee-sqbits/stack/1

Author: jglee-sqbits

max/python/max/pipelines/lib/diffusion_schedulers/__init__.py

@@ -15,3 +15,4 @@
 from .scheduling_flow_match_euler_discrete import (
     FlowMatchEulerDiscreteScheduler,
 )
+from .scheduling_unipc_multistep import UniPCMultistepScheduler

max/python/max/pipelines/lib/diffusion_schedulers/scheduler_factory.py

@@ -18,9 +18,11 @@
 from .scheduling_flow_match_euler_discrete import (
     FlowMatchEulerDiscreteScheduler,
 )
+from .scheduling_unipc_multistep import UniPCMultistepScheduler
 
 _SCHEDULER_REGISTRY: dict[str, type] = {
     "FlowMatchEulerDiscreteScheduler": FlowMatchEulerDiscreteScheduler,
+    "UniPCMultistepScheduler": UniPCMultistepScheduler,
 }