fix: implement IDSet (registryEntryHolderSet) decoder/encoder

- Replace old broken RegistryEntryHolderSetDecoder with correct varint
  discriminator: id != 0 returns raw ID, id == 0 decodes string base
- Add RegistryEntryHolderSetEncoder using type switch (string/int32)
- Add NonErroringIndex to proto_base (zero-fallback index lookup)
- DeclareRecipes now fails with index not found (mapper) instead of to do

mc/proto_base/base_types.go

@@ -262,6 +262,12 @@ func ErroringIndex[K comparable, V any, M map[K]V](m M, i K) (v V, err error) {
 	return
 }
 
+// NonErroringIndex looks up key k in map m and returns the zero value without error if not found.
+func NonErroringIndex[K comparable, V any, M map[K]V](m M, i K) (v V, err error) {
+	v, _ = m[i]
+	return
+}
+
 // EncodeDecodeAble is implemented by types that can be encoded and decoded for
 // Minecraft protocol packets.
 type EncodeDecodeAble interface {