feat: add download script for pre-built library

Adds scripts/download-lib.sh to download pre-built libmlx from
GitHub releases. Supports:
- macOS ARM64 (Metal)
- macOS x64 (CPU)
- Linux x64 (CPU)
- Linux ARM64 (CPU)
- Windows x64 (CPU)

Usage: ./scripts/download-lib.sh [version]

Author: zeekay

scripts/download-lib.sh

@@ -0,0 +1,62 @@
+#!/bin/bash
+# Download pre-built MLX library for Go bindings
+# Usage: ./scripts/download-lib.sh [version]
+
+set -e
+
+VERSION="${1:-latest}"
+REPO="luxfi/mlx"
+LIB_DIR="$(dirname "$0")/../lib"
+
+# Detect platform
+OS=$(uname -s | tr '[:upper:]' '[:lower:]')
+ARCH=$(uname -m)
+
+case "$OS" in
+    darwin)
+        case "$ARCH" in
+            arm64) PLATFORM="macos-arm64" ;;
+            x86_64) PLATFORM="macos-x64" ;;
+            *) echo "Unsupported macOS architecture: $ARCH"; exit 1 ;;
+        esac
+        ;;
+    linux)
+        case "$ARCH" in
+            x86_64) PLATFORM="linux-x64" ;;
+            aarch64) PLATFORM="linux-arm64" ;;
+            *) echo "Unsupported Linux architecture: $ARCH"; exit 1 ;;
+        esac
+        ;;
+    mingw*|msys*|cygwin*)
+        PLATFORM="windows-x64"
+        ;;
+    *)
+        echo "Unsupported OS: $OS"
+        exit 1
+        ;;
+esac
+
+FILENAME="libmlx-${PLATFORM}.tar.gz"
+
+echo "=== Downloading MLX library for $PLATFORM ==="
+mkdir -p "$LIB_DIR"
+
+if [ "$VERSION" = "latest" ]; then
+    URL="https://github.com/${REPO}/releases/latest/download/${FILENAME}"
+else
+    URL="https://github.com/${REPO}/releases/download/${VERSION}/${FILENAME}"
+fi
+
+echo "Downloading from: $URL"
+curl -fsSL "$URL" -o "/tmp/${FILENAME}" || {
+    echo "Failed to download. Available releases:"
+    echo "  https://github.com/${REPO}/releases"
+    exit 1
+}
+
+echo "Extracting to: $LIB_DIR"
+tar -xzf "/tmp/${FILENAME}" -C "$LIB_DIR"
+rm "/tmp/${FILENAME}"
+
+echo "=== MLX library installed successfully ==="
+ls -la "$LIB_DIR"