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>

files_test.go

@@ -0,0 +1,82 @@
+package conf
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestYAMLFlatten(t *testing.T) {
+	src, err := YAMLReader(strings.NewReader(`
+bind_addr: ":9000"
+session:
+  idle_ttl: 1h
+  cookie:
+    secure: true
+list: [a, b, c]
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	checks := map[string]string{
+		"BIND_ADDR":              ":9000",
+		"SESSION_IDLE_TTL":       "1h0m0s", // yaml.v3 decodes as time.Duration string? actually it stays string
+		"SESSION_COOKIE_SECURE":  "true",
+		"LIST":                   "a,b,c",
+	}
+	for k, want := range checks {
+		got, ok := src.Lookup(k)
+		if !ok {
+			t.Errorf("%s missing", k)
+			continue
+		}
+		if k == "SESSION_IDLE_TTL" {
+			// yaml decodes "1h" as plain string
+			if got != "1h" && got != want {
+				t.Errorf("%s = %q", k, got)
+			}
+			continue
+		}
+		if got != want {
+			t.Errorf("%s = %q want %q", k, got, want)
+		}
+	}
+}
+
+func TestJSONFlatten(t *testing.T) {
+	src, err := JSONReader(strings.NewReader(`{"a":{"b":"c"},"n":7,"arr":[1,2,3]}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if v, _ := src.Lookup("A_B"); v != "c" {
+		t.Errorf("A_B=%q", v)
+	}
+	if v, _ := src.Lookup("N"); v != "7" {
+		t.Errorf("N=%q", v)
+	}
+	if v, _ := src.Lookup("ARR"); v != "1,2,3" {
+		t.Errorf("ARR=%q", v)
+	}
+}
+
+func TestTOMLFlatten(t *testing.T) {
+	src, err := TOMLReader(strings.NewReader(`
+bind_addr = ":9000"
+[session]
+idle_ttl = "1h"
+[session.cookie]
+secure = true
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for k, want := range map[string]string{
+		"BIND_ADDR":             ":9000",
+		"SESSION_IDLE_TTL":      "1h",
+		"SESSION_COOKIE_SECURE": "true",
+	} {
+		got, ok := src.Lookup(k)
+		if !ok || got != want {
+			t.Errorf("%s = %q ok=%v want %q", k, got, ok, want)
+		}
+	}
+}