Add Intern-S2-Mobius (interns2_mobius) model

Hybrid Gated-DeltaNet / full-attention MoE with globally shared expert
banks, fused gate/up SwitchGLU, and a guarded Metal router-topk kernel
with a pure-MLX fallback. Includes a test_models entry covering both
cache types and a router kernel/ops A/B test.

Author: nightscape