Add reflective struct-tag config loader

Implements conf.Load to populate tagged structs from a chain of Sources
(env, .env, YAML/JSON/TOML, custom). Supports default values, slice
separators, nested structs, pointer fields, and a Validator hook.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

example_test.go

@@ -0,0 +1,46 @@
+package conf_test
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"git.juancwu.dev/juancwu/conf"
+	"git.juancwu.dev/juancwu/errx"
+)
+
+type Config struct {
+	BindAddr            string        `env:"BIND_ADDR" default:":8080"`
+	DatabaseURL         string        `env:"DATABASE_URL"`
+	SessionCookieSecure bool          `env:"SESSION_COOKIE_SECURE" default:"true"`
+	SessionIdleTTL      time.Duration `env:"SESSION_IDLE_TTL" default:"24h"`
+	JWTSecret           []byte        `env:"JWT_SECRET"`
+	WorkerConcurrency   int           `env:"WORKER_CONCURRENCY" default:"4"`
+}
+
+func (c *Config) Validate() error {
+	const op = "config.Validate"
+	if strings.TrimSpace(c.DatabaseURL) == "" {
+		return errx.New(op, "DATABASE_URL is required")
+	}
+	if len(c.JWTSecret) == 0 {
+		return errx.New(op, "JWT_SECRET is required")
+	}
+	return nil
+}
+
+func ExampleLoad() {
+	src := conf.MapSource(map[string]string{
+		"DATABASE_URL": "postgres://localhost/x",
+		"JWT_SECRET":   "shh",
+	})
+
+	var cfg Config
+	if err := conf.Load(&cfg, src); err != nil {
+		fmt.Println("err:", err)
+		return
+	}
+	fmt.Printf("bind=%s workers=%d idle=%s secure=%v",
+		cfg.BindAddr, cfg.WorkerConcurrency, cfg.SessionIdleTTL, cfg.SessionCookieSecure)
+	// Output: bind=:8080 workers=4 idle=24h0m0s secure=true
+}