Add signed invites, hang links, multi-share, and EXTENSIONS.md

- Signed invites: waste: URI gains inviter+sig fields (Ed25519); hello
  carries the invite so receiving peers can verify against known keys
- RequireInvite per-network flag: rejects peers without valid signed invite
- Hash-based hang links: #waste:base64 fragment pre-fills join form without
  server-side leakage of network name
- Multi-share: shares.json (daemon) + waste_shares localStorage (browser);
  IPC add_share/remove_share/list_shares commands
- EXTENSIONS.md: addendum documenting all waste-go protocol deviations from
  YAW/2; all extensions are additive and backward compatible

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

internal/mesh/peer.go

@@ -14,6 +14,7 @@ import (
 	"github.com/pion/webrtc/v3"
 
 	"github.com/waste-go/internal/crypto"
+	"github.com/waste-go/internal/invite"
 	"github.com/waste-go/internal/proto"
 )
 
@@ -43,11 +44,12 @@ func WireDataChannel(
 		localFP, remoteFP := dtlsFingerprints(pc)
 		bindBytes := proto.HelloBindString(localFP, remoteFP)
 		hello := proto.HelloMessage{
-			Type: "hello",
-			ID:   string(id.PeerID()),
-			Nick: id.Alias,
-			Caps: []string{"chat", "file"},
-			Sig:  id.Sign(bindBytes),
+			Type:   "hello",
+			ID:     string(id.PeerID()),
+			Nick:   id.Alias,
+			Caps:   []string{"chat", "file"},
+			Sig:    id.Sign(bindBytes),
+			Invite: m.InviteString,
 		}
 		helloJSON, _ := json.Marshal(hello)
 		if err := dc.SendText(string(helloJSON)); err != nil {
@@ -149,6 +151,37 @@ func handleDCMessage(data []byte, from proto.PeerID, localID *crypto.Identity, m
 			log.Printf("peer: bad hello from %s: %v", from.Short(), err)
 			return
 		}
+
+		// Invite enforcement (waste-go extension).
+		if m.RequireInvite {
+			if hello.Invite == "" {
+				log.Printf("peer: rejecting %s — no invite presented (RequireInvite=true)", from.Short())
+				m.Emit(proto.IpcMessage{
+					Type:         proto.EvtError,
+					ErrorMessage: fmt.Sprintf("peer %s rejected: no invite", from.Short()),
+				})
+				return
+			}
+			inv, err := invite.Decode(hello.Invite)
+			if err != nil || !inv.IsSigned() {
+				log.Printf("peer: rejecting %s — invite not signed: %v", from.Short(), err)
+				m.Emit(proto.IpcMessage{
+					Type:         proto.EvtError,
+					ErrorMessage: fmt.Sprintf("peer %s rejected: invite not signed", from.Short()),
+				})
+				return
+			}
+			trusted := m.trustedPeerIDs()
+			if err := invite.Verify(inv, trusted, crypto.Verify); err != nil {
+				log.Printf("peer: rejecting %s — %v", from.Short(), err)
+				m.Emit(proto.IpcMessage{
+					Type:         proto.EvtError,
+					ErrorMessage: fmt.Sprintf("peer %s rejected: %v", from.Short(), err),
+				})
+				return
+			}
+		}
+
 		// Update alias once we have the verified nick.
 		m.mu.Lock()
 		if conn, ok := m.peers[from]; ok {