net: exchange signed .btf descriptors between peers (#32)

Discovery has depended entirely on the Nostr relays. Two connected nodes never
told each other who else existed, so a relay outage left a running network
unable to grow, and a node that found one stale peer had no way to learn there
was anything better. The old `addr` gossip went out with the IRC seed.

Peers now trade signed descriptors over a new "btfpeers" message: our own
first, then peers that actually answered us. The receiver checks each Schnorr
signature against the key the `.btf` address decodes to, so the sender is
trusted for nothing -- forging an entry needs somebody else's secret key. That
leaves flooding with self-generated keys, which the per-message caps and the
"peers that answered" ordering are there to blunt.

Unknown commands are ignored (main.cpp), so nodes without this just drop the
message; nothing here is a protocol break.

The cache from the peer-cache change now keeps each peer's descriptor verbatim
alongside the parsed fields, because that signature is what makes a peer
relayable. Entries without one -- resolved through Nostr, or written before
this -- stay dialable, just not passed on. A dial never clears a descriptor
already on file.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/nostr.h b/src/nostr.h
index 04149e3..b5eb5e3 100644
--- a/src/nostr.h
+++ b/src/nostr.h
@@ -82,6 +82,14 @@ bool BtfGetIdentity(unsigned char pubkey[32], unsigned char enc_sk[32]);
 // This node's printable .btf address, or "" if the identity failed to load.
 std::string BtfLocalAddress();
 
+// This node's own signed descriptor, ready to hand to a peer over the wire.
+// "" until ThreadBtfAccept has registered at a rendezvous.
+std::string BtfLocalDescriptor();
+
+// The shared secp256k1 context, for btf::VerifyDescriptor (which takes it as
+// void* to keep headers free of secp). NULL if the identity failed to load.
+void* BtfSecpContext();
+
 // Resolve a .btf address via the public discovery relays: fetch the owner's
 // self-certified descriptor and return its meeting node ("host:port") and
 // x25519 public key for the end-to-end channel.