Add Vulkan matmul profiling instrumentation

[rasbid]

Summary

• add optional Vulkan profiling controlled by GGML_VK_PROFILING to capture per-dispatch timings for matmul and matmul_vec kernels
• collect cached pipeline executable statistics when available and surface them in logs and optional JSON output
• document usage instructions and mark the RX580 instrumentation plan tasks as complete

Testing

• not run (not requested)

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https://chatgpt.com/codex/tasks/task_e_68e4bc52b0b883309c2f6b7e9a76fdae