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.

cmd/regionio/main.go

@@ -0,0 +1,60 @@
+// Command regionio starts a RegionIO Minecraft server core.
+package main
+
+import (
+	"context"
+	"flag"
+	"log/slog"
+	"os"
+	"os/signal"
+	"strconv"
+	"syscall"
+
+	"regionio/internal/network"
+	"regionio/internal/server"
+)
+
+func main() {
+	log := slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
+		Level: slog.LevelDebug,
+	}))
+
+	cfg := server.DefaultConfig()
+
+	// World seed: -seed flag takes precedence, then REGIONIO_SEED env, then the
+	// default (0). Accepted formats: decimal, or "0x" hex. An invalid value is
+	// fatal — a wrong seed silently generates a different world than intended.
+	seedFlag := flag.Int64("seed", parseSeedEnv(os.Getenv("REGIONIO_SEED"), cfg.WorldSeed, log),
+		"world seed (overrides REGIONIO_SEED)")
+	flag.Parse()
+	cfg.WorldSeed = *seedFlag
+	log.Info("using world seed", "seed", cfg.WorldSeed)
+
+	srv := server.New(cfg)
+	ln := network.NewListener(srv, log)
+
+	// Cancel the context on SIGINT/SIGTERM for a graceful shutdown.
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	if err := ln.ListenAndServe(ctx); err != nil {
+		log.Error("server stopped", "err", err)
+		os.Exit(1)
+	}
+}
+
+// parseSeedEnv parses the REGIONIO_SEED env var. It returns fallback when the
+// variable is empty, and logs+returns fallback when parsing fails (so a typo
+// does not silently change the world).
+func parseSeedEnv(raw string, fallback int64, log *slog.Logger) int64 {
+	if raw == "" {
+		return fallback
+	}
+	// strconv.ParseInt with base 0 handles decimal and "0x" hex prefixes.
+	v, err := strconv.ParseInt(raw, 0, 64)
+	if err != nil {
+		log.Error("invalid REGIONIO_SEED, falling back to default", "raw", raw, "err", err)
+		return fallback
+	}
+	return v
+}