feat(backends): Native Windows jax-md support via Winloop

[ericchansen]

Summary

Enable native Windows installation of jax-md by using Winloop as a drop-in replacement for uvloop.

Problem

jax-md cannot be installed on Windows due to this transitive dependency chain:

jax-md -> flax -> orbax-checkpoint -> uvloop

uvloop is Unix-only (it wraps libuv's Unix event loop). This blocks native Windows usage of jax-md and forces WSL2 as a workaround (#92 phase 1).

Proposed solution

Winloop is a Windows-compatible fork of uvloop built on top of libuv's Windows IOCP backend. It provides a drop-in replacement API.

Options (in order of preference)

1. Upstream fix in orbax-checkpoint: Make uvloop an optional/conditional dependency -- only require it on non-Windows platforms. This is the right fix since orbax-checkpoint likely only uses uvloop for async checkpointing performance, not correctness.

2. Upstream fix in jax-md: Make flax an optional dependency (see feat(backends): Native Windows jax-md support via Winloop #94), eliminating the chain entirely for MM-only usage.

3. Local workaround: Install jax-md with --no-deps, then manually install dependencies substituting Winloop for uvloop:

   pip install winloop
   pip install jax-md --no-deps
   pip install flax --no-deps
   pip install orbax-checkpoint --no-deps
   # ... install remaining deps manually

4. Platform-conditional dependency in our pyproject.toml:

   [project.optional-dependencies]
   jax-md = [
       "jax-md; sys_platform != 'win32'",
   ]
   jax-md-win = [
       "jax-md; sys_platform == 'win32'",
       "winloop; sys_platform == 'win32'",
   ]

Acceptance criteria

• pip install q2mm[jax-md] works on Windows without WSL2
• JaxMDEngine passes all tests on native Windows
• CI includes a Windows + jax-md test matrix entry

Related issues

• Blocked by: jax-md integration (feat(backends): Add JAX differentiable MM backend with analytical gradients #92 or whichever tracks it)
• Related: feat(backends): Native Windows jax-md support via Winloop #94 (modular jax-md deps would eliminate this issue entirely)