Centralize D&D candidate JSON codecs

internal/modules/dnd/codec/candidatejson/candidate_json.go

@@ -0,0 +1,38 @@
+// Package candidatejson provides strict JSON mechanics for D&D artifact
+// candidates before artifact-specific validation.
+package candidatejson
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+)
+
+// EncodeCandidate encodes a typed candidate with an artifact-specific error
+// label.
+func EncodeCandidate[T any](label string, value T) ([]byte, error) {
+	content, err := json.Marshal(value)
+	if err != nil {
+		return nil, fmt.Errorf("encode %s: %w", label, err)
+	}
+	return content, nil
+}
+
+// DecodeCandidate decodes exactly one typed JSON value while rejecting unknown
+// fields and trailing values with an artifact-specific error label.
+func DecodeCandidate[T any](label string, content []byte) (T, error) {
+	decoder := json.NewDecoder(bytes.NewReader(content))
+	decoder.DisallowUnknownFields()
+	var value T
+	if err := decoder.Decode(&value); err != nil {
+		var zero T
+		return zero, fmt.Errorf("decode %s: %w", label, err)
+	}
+	var trailing any
+	if err := decoder.Decode(&trailing); err != io.EOF {
+		var zero T
+		return zero, fmt.Errorf("decode %s: multiple JSON values", label)
+	}
+	return value, nil
+}