sycl : fused MoE mul_mat_vec_q for TG (#21920)

sycl : fused MoE mul_mat_vec_q for TG (#21920)

* sycl : fused MoE mul_mat_vec_q for TG

Create an MMVQ kernel so ggml_sycl_mul_mat_id can consolidate
n_experts_used matmuls in a single kernel launch. The kernel
also reads expert IDs directly, removing a per-call host sync.

This is similar to the CUDA backend's ggml_cuda_mul_mat_vec_q*
paths.

All types supported in the current MMVQ are supported here as well:
Q2_K, Q3_K, Q4_K, Q5_K, Q6_K, Q4_0, Q4_1, Q5_0, Q5_1, Q8_0

It will fall back to the existing per-expert path when src0 has been rewritten
by opt_for_reorder(), and for any shape the fused path doesn't handle.

test-backend-ops passes for supported type/shape combos.

Benchmark: Qwen3-Next-35B-A3B Q4_K_M on Intel Arc B70 (SYCL0),
baseline 707c0b7a6, 16k context, -fa 0.

  build/bin/llama-bench -hf unsloth/Qwen3.5-35B-A3B-GGUF:Q4_K_M \
    -p 1024 -n 128 -d 16384 -ngl 99 -fa 0 -ub 2048 -r 2 -dev SYCL0

Before (3 runs on 707c0b7a6):

  | test            |            run 1 |            run 2 |            run 3 |
  | --------------- | ----------------:| ----------------:| ----------------:|
  | pp1024 @ d16384 |   533.26 ±  4.87 |   535.20 ±  2.78 |   524.27 ±  3.10 |
  | tg128  @ d16384 |    33.47 ±  0.02 |    33.31 ±  0.02 |    33.17 ±  0.05 |

After (3 runs on 707c0b7a6 + this patch):

  | test            |            run 1 |            run 2 |            run 3 |
  | --------------- | ----------------:| ----------------:| ----------------:|
  | pp1024 @ d16384 |   534.06 ±  0.97 |   531.95 ±  0.02 |   520.94 ± 20.10 |
  | tg128  @ d16384 |    45.85 ±  0.21 |    45.95 ±  0.45 |    46.22 ±  0.12 |

disclosure: Claude wrote it, but I reviewed and understand the implementation
(albeit my C is a little rusty).

* sycl: also support nvfp4 and mxfp4 expert types

* sycl: terser comments/nested dispatch in response to review

* sycl: more comment cleanup in mmvq.cpp/hpp

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Co-authored-by: Debian <redacted>