nostr: suppress retraversal of cross-FMP-version peers

Open-discovery NAT traversal succeeds at the UDP layer regardless
of what FMP-protocol version the peer speaks. When the daemon
discovers a peer running a different FMP version (e.g. a v0/v1 mix
during a mid-rollout window, or a misconfigured peer in the same
advert namespace), the punch sequence completes, the socket is
adopted via `Node::adopt_established_traversal`, and we initiate
an FMP handshake. The peer drops our msg1 at its own version-gate
and we drop their msg1/msg2 at `Unknown FMP version, dropping`.
Neither side advances the handshake.

Today the bootstrap transport sits idle until the 31s stale-
handshake timeout, drops, and the open-discovery sweep ~30s later
fires the full STUN+offer+answer+punch sequence again — every
minute, indefinitely, against peers the handshake literally cannot
complete with.

Add a `Node::bootstrap_transport_npubs` map populated alongside
`bootstrap_transports` at adopt time. The rx loop reverse-maps the
transport_id → npub on version-mismatch and bumps the discovery
layer's `failure_state` to a long structural cooldown via the new
`NostrDiscovery::record_protocol_mismatch` API. The next sweep
skips the npub for `protocol_mismatch_cooldown_secs` (default
86400 = 24h, separate from the 30-min transient-failure
`extended_cooldown_secs`).

One-shot WARN per fresh observation. Repeat mismatches inside the
cooldown window are silent (the failure_state method returns false
when an existing comparable cooldown is already in place). The
handshake/transport teardown chain is unchanged — the fix is
specifically about preventing the *next* sweep cycle from
re-traversing.

Cleared on `cleanup_bootstrap_transport_if_unused` and on the
adopt-failure rollback path so completed handshakes don't leave
stale entries behind.

Four new unit tests in `failure_state.rs` cover fresh-entry
signaling, repeat-suppression inside the window, streak-pin
behavior for `show_peers` rendering, and post-cooldown re-arming.

src/node/lifecycle.rs

@@ -1852,6 +1852,8 @@ impl Node {
             crate::transport::TransportHandle::Udp(transport),
         );
         self.bootstrap_transports.insert(transport_id);
+        self.bootstrap_transport_npubs
+            .insert(transport_id, traversal.peer_npub.clone());
 
         let remote_addr = TransportAddr::from_string(&traversal.remote_addr.to_string());
         if let Err(err) = self
@@ -1859,6 +1861,7 @@ impl Node {
             .await
         {
             self.bootstrap_transports.remove(&transport_id);
+            self.bootstrap_transport_npubs.remove(&transport_id);
             if let Some(mut handle) = self.transports.remove(&transport_id) {
                 let _ = handle.stop().await;
             }