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>

flatten.go

@@ -0,0 +1,44 @@
+package conf
+
+import (
+	"fmt"
+	"strings"
+)
+
+// flatten walks a nested map[string]any (as produced by yaml/json/toml decoders)
+// and returns a flat key->string map. Keys are uppercased and joined with "_".
+// Scalar values are stringified via fmt.Sprint. Slices are joined with ",".
+func flatten(in map[string]any) map[string]string {
+	out := map[string]string{}
+	flattenInto(out, "", in)
+	return out
+}
+
+func flattenInto(out map[string]string, prefix string, in map[string]any) {
+	for k, v := range in {
+		key := strings.ToUpper(k)
+		if prefix != "" {
+			key = prefix + "_" + key
+		}
+		switch x := v.(type) {
+		case map[string]any:
+			flattenInto(out, key, x)
+		case map[any]any: // yaml.v3 with non-string keys
+			m := make(map[string]any, len(x))
+			for kk, vv := range x {
+				m[fmt.Sprint(kk)] = vv
+			}
+			flattenInto(out, key, m)
+		case []any:
+			parts := make([]string, len(x))
+			for i, e := range x {
+				parts[i] = fmt.Sprint(e)
+			}
+			out[key] = strings.Join(parts, ",")
+		case nil:
+			// skip
+		default:
+			out[key] = fmt.Sprint(x)
+		}
+	}
+}