Migrate to YAW/2 protocol: WebRTC transport, anchor signaling, nacl/box crypto

Replace the original raw-TCP peer connections and Blowfish/RSA-era design with
the YAW/2 protocol stack:

Transport
- internal/mesh/peer.go: raw TCP + manual ECDH replaced with pion/webrtc
  DataChannels (ICE, DTLS, SCTP). Peers negotiate via sealed offer/answer/
  candidate exchange rather than direct dialling.
- internal/nat: deleted — ICE subsumes everything this package was going to do.

Signaling
- cmd/anchor: new WebSocket signaling server (replaces cmd/relay). Verifies
  Ed25519 challenge/join signatures, routes opaque nacl/box blobs, broadcasts
  peer-join/peer-leave events. Never sees plaintext signaling content.
- internal/anchor: new anchor client. Manages PeerConnection lifecycle,
  decides offerer by peer-id comparison, seals/opens signaling payloads with
  nacl/box, implements mesh.Anchor.

Crypto (internal/crypto)
- PeerID encoding: base64url → lowercase hex (64 chars, YAW/2 §2).
- Sign/Verify: hex signatures throughout.
- Added CurvePublicKey/CurvePrivateKey: X25519 keys derived from Ed25519
  identity via Montgomery conversion (filippo.io/edwards25519), matching
  libsodium's crypto_sign_ed25519_*_to_curve25519.
- Added SignalingBox/SignalingOpen: nacl/box (XSalsa20-Poly1305) seal/open
  for signaling payloads.

Wire types (internal/proto)
- ChatMessage gains Mid (random 16-byte hex) for mesh deduplication (§8).
- FileChunk removed from PeerMessage — chunks go on a separate binary
  DataChannel labelled "f:<xid>"; FileDone added.
- New types: SignalingPayload, AnchorMessage, HelloMessage, HelloBindString.
- PeerID.Short() now returns first 16 hex chars grouped in 4s.
- IPC: CmdConnect removed; CmdJoinNetwork/CmdLeaveNetwork added;
  EvtSessionReady added (fires when DataChannel opens and hello is received).

IPC (internal/ipc)
- join_network/leave_network replace connect (direct TCP dial).
- Network joins are context-scoped: leave_network or client disconnect
  cancels the anchor connection cleanly.

README updated to reflect new project layout, getting-started commands,
IPC protocol, and roadmap state.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/crypto/crypto_test.go

@@ -0,0 +1,71 @@
+package crypto
+
+import (
+	"encoding/hex"
+	"strings"
+	"testing"
+)
+
+func TestPeerIDIsHex(t *testing.T) {
+	id := newTestIdentity(t)
+	peerID := string(id.PeerID())
+
+	if len(peerID) != 64 {
+		t.Fatalf("PeerID length = %d, want 64", len(peerID))
+	}
+	if strings.ToLower(peerID) != peerID {
+		t.Fatal("PeerID is not lowercase")
+	}
+	if _, err := hex.DecodeString(peerID); err != nil {
+		t.Fatalf("PeerID is not valid hex: %v", err)
+	}
+}
+
+func TestSignVerifyHex(t *testing.T) {
+	id := newTestIdentity(t)
+	msg := []byte("test message")
+
+	sig := id.Sign(msg)
+	if err := Verify(string(id.PeerID()), msg, sig); err != nil {
+		t.Fatalf("Verify failed: %v", err)
+	}
+}
+
+func TestSignalingBoxRoundTrip(t *testing.T) {
+	alice := newTestIdentity(t)
+	bob := newTestIdentity(t)
+
+	plaintext := []byte(`{"kind":"offer","sdp":"v=0..."}`)
+
+	sealed := SignalingBox(plaintext, bob.CurvePublicKey(), alice.CurvePrivateKey())
+	got, err := SignalingOpen(sealed, alice.CurvePublicKey(), bob.CurvePrivateKey())
+	if err != nil {
+		t.Fatalf("SignalingOpen failed: %v", err)
+	}
+	if string(got) != string(plaintext) {
+		t.Fatalf("got %q, want %q", got, plaintext)
+	}
+}
+
+func TestSignalingBoxTamperedFails(t *testing.T) {
+	alice := newTestIdentity(t)
+	bob := newTestIdentity(t)
+
+	sealed := SignalingBox([]byte("hello"), bob.CurvePublicKey(), alice.CurvePrivateKey())
+
+	// wrong sender public key
+	carol := newTestIdentity(t)
+	if _, err := SignalingOpen(sealed, carol.CurvePublicKey(), bob.CurvePrivateKey()); err == nil {
+		t.Fatal("expected error with wrong sender key, got nil")
+	}
+}
+
+// newTestIdentity creates a fresh in-memory identity for testing.
+func newTestIdentity(t *testing.T) *Identity {
+	t.Helper()
+	id, err := LoadOrCreate(t.TempDir(), "test")
+	if err != nil {
+		t.Fatalf("LoadOrCreate: %v", err)
+	}
+	return id
+}