Windows build: generate the OpenCL import library, drop the SDK dependency

Building the `gpu` (OpenCL) backend on Windows no longer needs a vendor OpenCL
SDK. `cl-sys` links `OpenCL` (#[link(name = "OpenCL")]); instead of requiring an
SDK-provided OpenCL.lib, build.rs now generates a vendor-neutral import library
at build time from windows/OpenCL.def (all 118 cl-sys exports) — lib.exe for
MSVC (located via the cc crate), dlltool for MinGW — and puts it on the link
search path. The real OpenCL.dll is supplied at runtime by the GPU driver.

build.rs no-ops on non-Windows targets and when the gpu feature is off, and
warns rather than panics if the toolchain tool is absent so `cargo check` still
works. Combined with the runtime-loaded (dlopen) CUDA/NVML, a Windows build now
needs zero external GPU libraries.

Add BUILD-windows.md (toolchain, build, crt-static packaging, runtime deps, CI)
and link it from the README. Verified the whole crate compiles for
x86_64-pc-windows-gnu (default features and gpu,cuda); Linux is unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

windows/OpenCL.def

@@ -0,0 +1,124 @@
+; OpenCL import library definition for jackpotminer's Windows build.
+; Generated from the cl-sys 0.4.3 exports so a vendor-neutral OpenCL.lib /
+; libOpenCL.a can be produced at build time without an OpenCL SDK. The actual
+; OpenCL.dll (ICD loader) is provided at runtime by the GPU driver.
+LIBRARY OpenCL
+EXPORTS
+    clBuildProgram
+    clCloneKernel
+    clCompileProgram
+    clCreateBuffer
+    clCreateCommandQueue
+    clCreateCommandQueueWithProperties
+    clCreateContext
+    clCreateContextFromType
+    clCreateFromGLBuffer
+    clCreateFromGLRenderbuffer
+    clCreateFromGLTexture
+    clCreateFromGLTexture2D
+    clCreateFromGLTexture3D
+    clCreateImage
+    clCreateImage2D
+    clCreateImage3D
+    clCreateKernel
+    clCreateKernelsInProgram
+    clCreatePipe
+    clCreateProgramWithBinary
+    clCreateProgramWithBuiltInKernels
+    clCreateProgramWithIL
+    clCreateProgramWithSource
+    clCreateSampler
+    clCreateSamplerWithProperties
+    clCreateSubBuffer
+    clCreateSubDevices
+    clCreateUserEvent
+    clEnqueueAcquireGLObjects
+    clEnqueueBarrier
+    clEnqueueBarrierWithWaitList
+    clEnqueueCopyBuffer
+    clEnqueueCopyBufferRect
+    clEnqueueCopyBufferToImage
+    clEnqueueCopyImage
+    clEnqueueCopyImageToBuffer
+    clEnqueueFillBuffer
+    clEnqueueFillImage
+    clEnqueueMapBuffer
+    clEnqueueMapImage
+    clEnqueueMarker
+    clEnqueueMarkerWithWaitList
+    clEnqueueMigrateMemObjects
+    clEnqueueNativeKernel
+    clEnqueueNDRangeKernel
+    clEnqueueReadBuffer
+    clEnqueueReadBufferRect
+    clEnqueueReadImage
+    clEnqueueReleaseGLObjects
+    clEnqueueSVMFree
+    clEnqueueSVMMap
+    clEnqueueSVMMemcpy
+    clEnqueueSVMMemFill
+    clEnqueueSVMMigrateMem
+    clEnqueueSVMUnmap
+    clEnqueueTask
+    clEnqueueUnmapMemObject
+    clEnqueueWaitForEvents
+    clEnqueueWriteBuffer
+    clEnqueueWriteBufferRect
+    clEnqueueWriteImage
+    clFinish
+    clFlush
+    clGetCommandQueueInfo
+    clGetContextInfo
+    clGetDeviceAndHostTimer
+    clGetDeviceIDs
+    clGetDeviceInfo
+    clGetEventInfo
+    clGetEventProfilingInfo
+    clGetExtensionFunctionAddress
+    clGetExtensionFunctionAddressForPlatform
+    clGetGLContextInfoKHR
+    clGetGLObjectInfo
+    clGetGLTextureInfo
+    clGetHostTimer
+    clGetImageInfo
+    clGetKernelArgInfo
+    clGetKernelInfo
+    clGetKernelSubGroupInfo
+    clGetKernelWorkGroupInfo
+    clGetMemObjectInfo
+    clGetPipeInfo
+    clGetPlatformIDs
+    clGetPlatformInfo
+    clGetProgramBuildInfo
+    clGetProgramInfo
+    clGetSamplerInfo
+    clGetSupportedImageFormats
+    clLinkProgram
+    clReleaseCommandQueue
+    clReleaseContext
+    clReleaseDevice
+    clReleaseEvent
+    clReleaseKernel
+    clReleaseMemObject
+    clReleaseProgram
+    clReleaseSampler
+    clRetainCommandQueue
+    clRetainContext
+    clRetainDevice
+    clRetainEvent
+    clRetainKernel
+    clRetainMemObject
+    clRetainProgram
+    clRetainSampler
+    clSetDefaultDeviceCommandQueue
+    clSetEventCallback
+    clSetKernelArg
+    clSetKernelArgSVMPointer
+    clSetKernelExecInfo
+    clSetMemObjectDestructorCallback
+    clSetUserEventStatus
+    clSVMAlloc
+    clSVMFree
+    clUnloadCompiler
+    clUnloadPlatformCompiler
+    clWaitForEvents