[metal] Add fused RMS_NORM + MUL + SWIGLU for Qwen3Next

[MemoryIt]

This PR adds a new fused operator in the Metal backend to support Qwen3NextRMSNormGated from Qwen3-Next models:

• Extends kernel_rms_norm_fuse_impl with F == 4: fuses rms_norm + mul + swiglu_split into a single kernel.
• Matches the behavior of Qwen3NextRMSNormGated in modeling_qwen3_next.py.
• Extends fusion detection logic to recognize SWIGLU_SPLIT as a fusible tail op.
• Expands test_rms_norm_mul_add test class to cover the new fusion pattern.
• ✅ Passes test-backend-ops with full parameter coverage (broadcast, eps, multi_add).
• ✅ Passes local CI on macOS (CPU + Metal backends, numerical consistency verified).

Tested on Apple M4 Pro. All tests pass.

Related to #15940