params: fix param value decoding

params/config2.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"maps"
 	"math/big"
+	"reflect"
 	"slices"
 	"strconv"
 	"strings"
@@ -171,7 +172,10 @@ func (cfg *Config2) UnmarshalJSON(input []byte) error {
 		return fmt.Errorf("expected JSON object for chain configuration")
 	}
 	// Now we're in the object.
-	newcfg := Config2{activation: Activations{}}
+	newcfg := Config2{
+		activation: make(Activations),
+		param:      make(map[int]any),
+	}
 	for {
 		tok, err = dec.Token()
 		if tok == json.Delim('}') {
@@ -227,7 +231,7 @@ func (cfg *Config2) decodeParameter(key string, dec *json.Decoder) error {
 	if err := dec.Decode(v); err != nil {
 		return err
 	}
-	cfg.param[id] = v
+	cfg.param[id] = reflect.ValueOf(v).Elem().Interface()
 	return nil
 }
 

params/registry.go

@@ -101,8 +101,7 @@ func Define[V any](def T[V]) Parameter[V] {
 		optional:     def.Optional,
 		defaultValue: def.Default,
 		new: func() any {
-			var z V
-			return z
+			return new(V)
 		},
 		validate: func(v any, cfg *Config2) error {
 			if def.Validate == nil {