[Fix][Relax][Metal] Constrain wide-head prefill tiling - #20235
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Apply the existing low-storage WebGPU prefill configuration to Metal for wide attention heads. This keeps generated threadgroup allocations below Metal device limits, including Gemma 4 global attention heads.
Choose Metal prefill tiling from the target shared-memory limit and the kernel's exact Q/K/V layout. Account for unequal ragged-attention dimensions and MLA's merged KV buffer, and reject configurations that do not satisfy the scheduler's factorization constraints.\n\nShare configuration and factorization helpers with both tree-attention paths while preserving existing WebGPU behavior. Add allocation-level coverage for standard, ragged, MLA, and tree-attention kernels, custom Metal limits, and impossible limits.
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Apply the existing low-storage WebGPU prefill configuration to Metal for wide attention heads. This keeps generated threadgroup allocations below Metal device limits, including Gemma 4 global attention heads.