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.go

@@ -0,0 +1,103 @@
+package conf
+
+import (
+	"bufio"
+	"io"
+	"os"
+	"strings"
+
+	"git.juancwu.dev/juancwu/errx"
+)
+
+// DotEnvFile loads KEY=VALUE pairs from path. Lines starting with # and blank
+// lines are ignored. Values may be wrapped in single or double quotes; double
+// quotes honor \n, \t, \r, \\, and \" escapes.
+func DotEnvFile(path string) (Source, error) {
+	const op = "conf.DotEnvFile"
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, errx.Wrapf(op, err, "open %s", path)
+	}
+	defer f.Close()
+	return DotEnvReader(f)
+}
+
+// DotEnvReader is DotEnvFile for an arbitrary reader.
+func DotEnvReader(r io.Reader) (Source, error) {
+	const op = "conf.DotEnvReader"
+
+	m := map[string]string{}
+	sc := bufio.NewScanner(r)
+	lineNo := 0
+	for sc.Scan() {
+		lineNo++
+		line := strings.TrimSpace(sc.Text())
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		line = strings.TrimPrefix(line, "export ")
+		eq := strings.IndexByte(line, '=')
+		if eq < 0 {
+			return nil, errx.Newf(op, "line %d: missing '='", lineNo)
+		}
+		key := strings.TrimSpace(line[:eq])
+		val := strings.TrimSpace(line[eq+1:])
+		// strip trailing inline comment for unquoted values
+		if !isQuoted(val) {
+			if i := strings.Index(val, " #"); i >= 0 {
+				val = strings.TrimSpace(val[:i])
+			}
+		}
+		v, err := unquote(val)
+		if err != nil {
+			return nil, errx.Wrapf(op, err, "line %d", lineNo)
+		}
+		m[key] = v
+	}
+	if err := sc.Err(); err != nil {
+		return nil, errx.Wrap(op, err)
+	}
+	return MapSource(m), nil
+}
+
+func isQuoted(s string) bool {
+	if len(s) < 2 {
+		return false
+	}
+	return (s[0] == '"' && s[len(s)-1] == '"') || (s[0] == '\'' && s[len(s)-1] == '\'')
+}
+
+func unquote(s string) (string, error) {
+	if len(s) >= 2 && s[0] == '\'' && s[len(s)-1] == '\'' {
+		return s[1 : len(s)-1], nil
+	}
+	if len(s) >= 2 && s[0] == '"' && s[len(s)-1] == '"' {
+		inner := s[1 : len(s)-1]
+		var b strings.Builder
+		for i := 0; i < len(inner); i++ {
+			c := inner[i]
+			if c != '\\' || i+1 >= len(inner) {
+				b.WriteByte(c)
+				continue
+			}
+			i++
+			switch inner[i] {
+			case 'n':
+				b.WriteByte('\n')
+			case 't':
+				b.WriteByte('\t')
+			case 'r':
+				b.WriteByte('\r')
+			case '\\':
+				b.WriteByte('\\')
+			case '"':
+				b.WriteByte('"')
+			default:
+				b.WriteByte('\\')
+				b.WriteByte(inner[i])
+			}
+		}
+		return b.String(), nil
+	}
+	return s, nil
+}