Metal backend: Add gated delta rule kernel for linear attention

[manuelcandales]

Adds Metal kernel for the gated delta rule recurrence used by Qwen 3.5
MoE's GatedDeltaNet linear attention layers. Ported from the MLX delegate
PR (#18785) Metal shader. The kernel processes the full sequence
sequentially within a single GPU dispatch, keeping recurrent state in
per-thread registers.

Grid: [32, Dv, B*Hv], Threadgroup: [32, 4, 1]. Each simdgroup of 32
threads handles Dk/32 elements of the key dimension with SIMD reduction
for dot products.

The op mutates the recurrent state buffer in-place (mutates_args).
Instantiated for both real model (Dk=128, Dv=128, Hk=32, Hv=32) and
tiny test (Dk=64, Dv=64, Hk=4, Hv=4) dimensions.

Includes: Metal shader + C++ host dispatch, Python custom op definition
(metal::gated_delta_rule) with reference CPU impl and Meta impl, C shim
dict, fallback kernel registration, CMakeLists entry, and test module.

Authored with Claude.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• Qwen 3.5 MoE: Add Metal build target #18881
• Qwen 3.5 MoE: Add --backend metal export path #18880
• Qwen 3.5 MoE: Add Metal source transformations #18879
• -> Metal backend: Add gated delta rule kernel for linear attention #18878
• Metal backend: Add gather_qmv kernel for MoE expert-indexed quantized matmul #18877
• Metal backend: Add topk fallback kernel via MPSGraph #18876
• Metal backend: Add SDPA head_dim=256 support #18875
• Metal backend: Add INT32 and BOOL dtype support #18874

[pytorch-bot]

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