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>

BUILD-windows.md

@@ -0,0 +1,119 @@
+# Building jackpotminer on Windows
+
+The miner builds on Windows with **no external GPU SDKs**. The CUDA driver and
+NVML are loaded at runtime (`dlopen`/`LoadLibrary`), and the OpenCL import
+library is generated at build time from `windows/OpenCL.def` — so you don't need
+the CUDA Toolkit, an OpenCL SDK, or any vendor libraries to compile. The whole
+codebase (miner + the `jackpotminer-config` GUI) is verified to compile for
+`x86_64-pc-windows-*`.
+
+## Prerequisites
+
+- **Rust** (https://rustup.rs). The default target is `x86_64-pc-windows-msvc`.
+- **MSVC build tools** — Visual Studio 2019/2022 or the standalone *Build Tools
+  for Visual Studio* with the **“Desktop development with C++”** workload (gives
+  `link.exe`, `lib.exe`, and the Windows SDK).
+  - Build from a **“x64 Native Tools Command Prompt for VS”** (or any shell where
+    the MSVC environment is active) so `lib.exe` is found. `cargo` also locates
+    it automatically via the registry in most cases.
+- *(Alternative toolchain)* the GNU target `x86_64-pc-windows-gnu` works too,
+  with **MinGW-w64** on `PATH` (provides `dlltool` and the linker).
+
+No CUDA Toolkit and **no OpenCL SDK** are required.
+
+## Build
+
+```powershell
+:: Default: OpenCL + CUDA backends + the GUI config tool
+cargo build --release
+
+:: Miner only (no GUI), both GPU backends
+cargo build --release --no-default-features --features gpu,cuda
+
+:: OpenCL only (AMD / Intel / NVIDIA)
+cargo build --release --no-default-features --features gpu
+
+:: NVIDIA only — needs nothing external at build time (CUDA is dlopen'd)
+cargo build --release --no-default-features --features cuda
+```
+
+Outputs: `target\release\jackpotminer.exe` (and `jackpotminer-config.exe` with
+the default features).
+
+### How the OpenCL build dependency is avoided
+
+`ocl`/`cl-sys` link `OpenCL` (`#[link(name = "OpenCL")]`), which normally needs
+an `OpenCL.lib` from a vendor SDK. Instead, `build.rs` generates a
+vendor-neutral import library from `windows/OpenCL.def`:
+
+- **MSVC:** `lib.exe /def:windows\OpenCL.def /out:OpenCL.lib /machine:X64`
+- **GNU:** `dlltool -d windows/OpenCL.def -l libOpenCL.a -m i386:x86-64`
+
+and puts it on the link search path. The import library only forwards to
+`OpenCL.dll`, which the GPU driver provides at runtime. If the toolchain tool
+isn’t on `PATH`, `build.rs` prints a warning and the link step fails with a clear
+message (compilation/`cargo check` still works).
+
+## Distribution
+
+Statically link the MSVC C runtime so users don’t need the VC++ redistributable:
+
+```powershell
+set RUSTFLAGS=-C target-feature=+crt-static
+cargo build --release
+```
+
+## Runtime dependencies (on the mining machine)
+
+- **`OpenCL.dll`** — the ICD loader, installed with any GPU driver (AMD/NVIDIA/
+  Intel). Required for the OpenCL backend.
+- **`nvcuda.dll`** + **`nvml.dll`** — installed with the NVIDIA driver. Loaded
+  on demand for the CUDA backend; absent on AMD-only machines, where the miner
+  simply reports no CUDA devices.
+- The **VC++ runtime**, unless you built with `+crt-static`.
+
+A `cuda`-enabled binary still starts on a machine with no NVIDIA driver.
+
+## Building Windows binaries without a Windows machine
+
+### GitHub Actions (recommended)
+
+```yaml
+name: windows
+on: [push, workflow_dispatch]
+jobs:
+  build:
+    runs-on: windows-latest          # MSVC + lib.exe already on PATH
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo build --release --features gpu,cuda
+        env:
+          RUSTFLAGS: -C target-feature=+crt-static
+      - uses: actions/upload-artifact@v4
+        with:
+          name: jackpotminer-windows
+          path: target/release/*.exe
+```
+
+### Cross-compile from Linux (GNU target)
+
+```bash
+rustup target add x86_64-pc-windows-gnu
+sudo pacman -S mingw-w64           # or your distro's mingw-w64 (gives dlltool + linker)
+cargo build --release --target x86_64-pc-windows-gnu --no-default-features --features gpu,cuda
+```
+
+The MSVC target can’t be linked from Linux. The GUI config tool (`eframe`) is
+easiest to build natively on Windows.
+
+## Status / caveats
+
+- **Compilation for Windows is verified** here via `cargo check
+  --target x86_64-pc-windows-gnu` (default features, and `gpu,cuda`).
+- The OpenCL **import-library linking** uses the standard `lib.exe`/`dlltool`
+  technique; validate it with an actual Windows (or MinGW cross) build, which
+  needs those tools present.
+- `relaunch_in_terminal` (reopen-in-a-terminal on GUI launch) is Linux-only;
+  harmless on Windows, where double-clicking a console binary already opens a
+  console.