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>

dotenv_test.go

@@ -0,0 +1,35 @@
+package conf
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestDotEnvReader(t *testing.T) {
+	in := `# comment
+FOO=bar
+export QUOTED="hello\nworld"
+SINGLE='ab cd'
+INLINE=plain # trailing
+EMPTY=
+`
+	src, err := DotEnvReader(strings.NewReader(in))
+	if err != nil {
+		t.Fatal(err)
+	}
+	cases := map[string]string{
+		"FOO":    "bar",
+		"QUOTED": "hello\nworld",
+		"SINGLE": "ab cd",
+		"INLINE": "plain",
+	}
+	for k, want := range cases {
+		got, ok := src.Lookup(k)
+		if !ok || got != want {
+			t.Errorf("%s = %q ok=%v, want %q", k, got, ok, want)
+		}
+	}
+	if _, ok := src.Lookup("EMPTY"); ok {
+		t.Errorf("EMPTY should be absent")
+	}
+}