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/mesh.go

@@ -27,8 +27,10 @@ type PeerConn struct {
 type Mesh struct {
 	Identity    *crypto.Identity
 	Store       *store.Store // may be nil if persistence is disabled
-	ShareDir    string       // directory whose contents are shared with peers; "" = no sharing
-	DownloadDir string       // directory where received files are saved
+	ShareDir      string // directory whose contents are shared with peers; "" = no sharing
+	DownloadDir   string // directory where received files are saved
+	RequireInvite bool   // waste-go ext: reject peers that present no valid signed invite
+	InviteString  string // the invite this peer used to join (sent in hello to other peers)
 	// ScanFiles overrides ScanShareDir when set — allows the manager to inject
 	// multi-share scanning without the mesh needing to know about shares.json.
 	ScanFiles func() []proto.FileEntry
@@ -63,6 +65,29 @@ func New(id *crypto.Identity, st *store.Store) *Mesh {
 	}
 }
 
+// trustedPeerIDs returns a set of peer IDs trusted on this network:
+// all currently connected peers plus all peers in the persistent store.
+func (m *Mesh) trustedPeerIDs() map[string]bool {
+	trusted := map[string]bool{}
+	// Own identity is always trusted.
+	trusted[string(m.Identity.PeerID())] = true
+	// Connected peers.
+	m.mu.RLock()
+	for id := range m.peers {
+		trusted[string(id)] = true
+	}
+	m.mu.RUnlock()
+	// Previously seen peers from the store.
+	if m.Store != nil {
+		if known, err := m.Store.KnownPeers(); err == nil {
+			for id := range known {
+				trusted[string(id)] = true
+			}
+		}
+	}
+	return trusted
+}
+
 // ScanShareDir returns the list of files in the local share directory.
 // Returns an empty slice if ShareDir is unset or the directory is empty.
 func (m *Mesh) ScanShareDir() []proto.FileEntry {