chore: initial commit for v0.0.1

DChain single-node blockchain + React Native messenger client.

Core:
- PBFT consensus with multi-sig validator admission + equivocation slashing
- BadgerDB + schema migration scaffold (CurrentSchemaVersion=0)
- libp2p gossipsub (tx/v1, blocks/v1, relay/v1, version/v1)
- Native Go contracts (username_registry) alongside WASM (wazero)
- WebSocket gateway with topic-based fanout + Ed25519-nonce auth
- Relay mailbox with NaCl envelope encryption (X25519 + Ed25519)
- Prometheus /metrics, per-IP rate limit, body-size cap

Deployment:
- Single-node compose (deploy/single/) with Caddy TLS + optional Prometheus
- 3-node dev compose (docker-compose.yml) with mocked internet topology
- 3-validator prod compose (deploy/prod/) for federation
- Auto-update from Gitea via /api/update-check + systemd timer
- Build-time version injection (ldflags → node --version)
- UI / Swagger toggle flags (DCHAIN_DISABLE_UI, DCHAIN_DISABLE_SWAGGER)

Client (client-app/):
- Expo / React Native / NativeWind
- E2E NaCl encryption, typing indicator, contact requests
- Auto-discovery of canonical contracts, chain_id aware, WS reconnect on node switch

Documentation:
- README.md, CHANGELOG.md, CONTEXT.md
- deploy/single/README.md with 6 operator scenarios
- deploy/UPDATE_STRATEGY.md with 4-layer forward-compat design
- docs/contracts/*.md per contract

relay/keypair.go

@@ -0,0 +1,54 @@
+package relay
+
+import (
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"os"
+)
+
+type keypairFile struct {
+	Pub  string `json:"pub"`
+	Priv string `json:"priv"`
+}
+
+// LoadOrCreateKeyPair loads an X25519 relay keypair from path, creating it if absent.
+func LoadOrCreateKeyPair(path string) (*KeyPair, error) {
+	data, err := os.ReadFile(path)
+	if err == nil {
+		var f keypairFile
+		if err := json.Unmarshal(data, &f); err != nil {
+			return nil, fmt.Errorf("parse relay key file: %w", err)
+		}
+		pubBytes, err := hex.DecodeString(f.Pub)
+		if err != nil || len(pubBytes) != 32 {
+			return nil, fmt.Errorf("invalid relay pub key in %s", path)
+		}
+		privBytes, err := hex.DecodeString(f.Priv)
+		if err != nil || len(privBytes) != 32 {
+			return nil, fmt.Errorf("invalid relay priv key in %s", path)
+		}
+		kp := &KeyPair{}
+		copy(kp.Pub[:], pubBytes)
+		copy(kp.Priv[:], privBytes)
+		return kp, nil
+	}
+	if !os.IsNotExist(err) {
+		return nil, fmt.Errorf("read relay key file: %w", err)
+	}
+
+	// Generate new keypair and persist.
+	kp, err := GenerateKeyPair()
+	if err != nil {
+		return nil, err
+	}
+	f := keypairFile{
+		Pub:  kp.PubHex(),
+		Priv: hex.EncodeToString(kp.Priv[:]),
+	}
+	out, _ := json.MarshalIndent(f, "", "  ")
+	if err := os.WriteFile(path, out, 0600); err != nil {
+		return nil, fmt.Errorf("write relay key file: %w", err)
+	}
+	return kp, nil
+}