fix: preserve Qwen3.5 MoE switch projection names

[ukint-vs]

Summary

• stack Qwen3.5 MoE per-expert gate_proj, up_proj, and down_proj weights under the switch_mlp.*_proj names read by SwitchGLU::from_weights
• remove the sanitizer rename that rewrote switch_mlp.gate_proj/up_proj/down_proj to w1/w3/w2
• add a focused sanitizer regression test for the per-expert checkpoint layout

Root cause

Some Qwen3.5 MoE checkpoints store expert weights as model.layers.N.mlp.experts.E.{gate_proj,up_proj,down_proj}. The sanitizer stacked those tensors but wrote them as switch_mlp.w1/w3/w2, while the loader later expects switch_mlp.gate_proj/up_proj/down_proj. That caused model loading to fail with missing switch_mlp.gate_proj weights.

Validation

• cargo test qwen3_5_tests::sanitize --features metal,accelerate -- --ignored --test-threads=1
• manually validated text and image generation with mlx-community/Ornith-1.0-35B-4bit after the loader fix

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