feat(config): load embedded defaults and merge optional overlay

internal/config/config.go

@@ -0,0 +1,82 @@
+// Package config loads embedded defaults and optionally merges a user
+// overlay from ~/.config/archduino/overlay.yaml.
+package config
+
+import (
+	_ "embed"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"gopkg.in/yaml.v3"
+)
+
+//go:embed defaults.yaml
+var defaultsYAML []byte
+
+// Config is the fully-resolved runtime configuration.
+type Config struct {
+	Version         int               `yaml:"version"`
+	ExtraPackages   []string          `yaml:"extra_packages"`
+	DisabledModules []string          `yaml:"disabled_modules"`
+	ThemeOverrides  map[string]string `yaml:"theme_overrides"`
+}
+
+const currentVersion = 1
+
+// Load returns defaults merged with the overlay at overlayPath (if present).
+// If overlayPath is "", the default location (~/.config/archduino/overlay.yaml)
+// is used. A missing overlay is not an error.
+func Load(overlayPath string) (Config, error) {
+	var cfg Config
+	if err := yaml.Unmarshal(defaultsYAML, &cfg); err != nil {
+		return Config{}, fmt.Errorf("parse embedded defaults: %w", err)
+	}
+
+	if overlayPath == "" {
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return cfg, nil
+		}
+		overlayPath = filepath.Join(home, ".config", "archduino", "overlay.yaml")
+	}
+
+	data, err := os.ReadFile(overlayPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return cfg, nil
+		}
+		return Config{}, fmt.Errorf("read overlay %s: %w", overlayPath, err)
+	}
+
+	var overlay Config
+	if err := yaml.Unmarshal(data, &overlay); err != nil {
+		return Config{}, fmt.Errorf("parse overlay %s: %w", overlayPath, err)
+	}
+	if overlay.Version != 0 && overlay.Version != currentVersion {
+		return Config{}, fmt.Errorf("overlay %s: unsupported version %d (want %d)", overlayPath, overlay.Version, currentVersion)
+	}
+
+	return merge(cfg, overlay), nil
+}
+
+func merge(base, over Config) Config {
+	out := base
+	if len(over.ExtraPackages) > 0 {
+		out.ExtraPackages = append(append([]string{}, base.ExtraPackages...), over.ExtraPackages...)
+	}
+	if len(over.DisabledModules) > 0 {
+		out.DisabledModules = append(append([]string{}, base.DisabledModules...), over.DisabledModules...)
+	}
+	if len(over.ThemeOverrides) > 0 {
+		merged := make(map[string]string, len(base.ThemeOverrides)+len(over.ThemeOverrides))
+		for k, v := range base.ThemeOverrides {
+			merged[k] = v
+		}
+		for k, v := range over.ThemeOverrides {
+			merged[k] = v
+		}
+		out.ThemeOverrides = merged
+	}
+	return out
+}