Initial commit: RegionIO Minecraft server core (26.1.2/protocol 775)

Vanilla-faithful overworld generator (final_density + multi-noise biomes),
full connection lifecycle (status/login/configuration/play), chunk streaming,
creative block editing, and the protocol/nbt/registry infrastructure.

internal/registry/registry.go

@@ -0,0 +1,79 @@
+// Package registry holds the set of synchronized registries the server sends
+// to the client during the configuration phase.
+//
+// The data in synced_registries.json was captured verbatim from the official
+// 26.1.2 server: 28 registries in exact send order, each entry flagged
+// has_data=false. Because we advertise the same "minecraft:core" known pack
+// that a matching client already has, the client fills in each entry's data
+// from its built-in pack, so we transmit only the entry identifiers. Entry
+// order is significant: it defines the network (numeric) IDs used later in the
+// play phase (biome IDs in chunks, dimension-type indices, and so on).
+package registry
+
+import (
+	_ "embed"
+	"encoding/json"
+	"fmt"
+)
+
+//go:embed synced_registries.json
+var syncedJSON []byte
+
+// syncedTags is the verbatim body of the vanilla 26.1.2 update_tags packet,
+// captured from the official server. Tags reference registry entries by their
+// numeric (network) index, so this blob is only valid because our synchronized
+// registries are sent in the exact same order vanilla uses.
+//
+//go:embed synced_tags.bin
+var syncedTags []byte
+
+// Tags returns the update_tags packet body. The slice is shared and must not
+// be mutated.
+func Tags() []byte { return syncedTags }
+
+// Registry is one synchronized registry and its ordered entry identifiers.
+type Registry struct {
+	Name    string   `json:"name"`
+	Entries []string `json:"entries"`
+}
+
+// synced is the parsed, ordered list loaded once at init.
+var synced []Registry
+
+func init() {
+	if err := json.Unmarshal(syncedJSON, &synced); err != nil {
+		panic(fmt.Sprintf("registry: parsing embedded synced_registries.json: %v", err))
+	}
+}
+
+// Synced returns the ordered synchronized registries. The slice is shared and
+// must not be mutated.
+func Synced() []Registry { return synced }
+
+// Index returns the zero-based position of entry within the named registry,
+// which is the numeric (network) ID the client assigns it. It returns -1 if
+// the registry or entry is unknown.
+func Index(registryName, entry string) int {
+	for _, reg := range synced {
+		if reg.Name != registryName {
+			continue
+		}
+		for i, e := range reg.Entries {
+			if e == entry {
+				return i
+			}
+		}
+	}
+	return -1
+}
+
+// KnownPack identifies a resource/data pack advertised via select_known_packs.
+type KnownPack struct {
+	Namespace string
+	ID        string
+	Version   string
+}
+
+// CorePack is the vanilla built-in pack. Advertising it lets a matching client
+// supply registry contents from its own copy.
+var CorePack = KnownPack{Namespace: "minecraft", ID: "core", Version: "26.1.2"}