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>

json.go

@@ -0,0 +1,33 @@
+package conf
+
+import (
+	"encoding/json"
+	"io"
+	"os"
+
+	"git.juancwu.dev/juancwu/errx"
+)
+
+// JSONFile loads a JSON object and exposes it as a flat key->string Source.
+func JSONFile(path string) (Source, error) {
+	const op = "conf.JSONFile"
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, errx.Wrapf(op, err, "open %s", path)
+	}
+	defer f.Close()
+	return JSONReader(f)
+}
+
+// JSONReader is JSONFile for an arbitrary reader.
+func JSONReader(r io.Reader) (Source, error) {
+	const op = "conf.JSONReader"
+	var raw map[string]any
+	if err := json.NewDecoder(r).Decode(&raw); err != nil {
+		if err == io.EOF {
+			return MapSource(nil), nil
+		}
+		return nil, errx.Wrap(op, err)
+	}
+	return MapSource(flatten(raw)), nil
+}