build: sync submodules to latest main (#90)

Updates mlx-swift and mlx-swift-lm submodules to pull in the stacked-buffer MoE fast-paths and memory-safety optimizations.

Submodule Updates:
- `mlx-swift`: SharpAI/mlx-swift#10
- `mlx-swift-lm`: SharpAI/mlx-swift-lm#35

All downstream integration tests pass.

Author: ericjlake