Match Qwen head-chunked attention execution

docs/issue113-head-chunk/qsa-module-0.log

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+
+running 1 test
+test engine::metal::qwen_mtplx::qsa_attention::mtplx_qsa_attention_module_matches_reference ... ds4: Metal device Apple M5 Max, 128.00 GiB RAM
+ds4: Metal 4 tensor API enabled for Tensor kernels
+ds4: drift-patch flags hc_stable=on norm_unify=on kv_raw_f32=off rope_exp2_log2=off math_safe=off tensor_matmul=on
+ds4: Qwen reference kernels: Powered by MTPLX — https://github.com/youssofal/mtplx
+ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 548 filtered out; finished in 0.94s