feat(federation): advertise own_fips_npub in state snapshots

Pre-v1.4 federation pairs (who exchanged invites before fips_npub
was part of the invite code) had no path to learn each other's FIPS
npub — they'd stay Tor-only forever even after upgrading. Fix:
every state snapshot now carries the sender's own_fips_npub, and
update_node_state refreshes the stored fips_npub on the receiver
side whenever it differs.

- NodeStateSnapshot.own_fips_npub (serde default for back-compat).
- build_local_state takes own_fips_npub alongside the other
  single-value fields.
- handle_federation_get_state populates own_fips_npub from
  identity::fips_npub, with a fallback to the upstream daemon's
  /etc/fips/fips.pub for legacy nodes that never materialised a
  seed-derived key.
- storage::update_node_state now writes fips_npub into the
  FederatedNode when a new value arrives and trims whitespace
  before comparing, so key rotations also flow through.
- Test fixtures (storage + transport/delta + sync) updated for the
  new field; existing tests pass.

Net effect: on the next sync, .116 and .228 learn each other's
fips_npub (currently null from the old invite) and subsequent
federation calls route FIPS-first automatically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/federation/handlers.rs

@@ -387,6 +387,23 @@ impl RpcHandler {
             .await
             .unwrap_or_default();
 
+        // Our own FIPS npub, so pre-v1.4 federation pairs (whose
+        // invite codes didn't carry it) can learn it on the next sync.
+        let identity_dir = self.config.data_dir.join("identity");
+        let own_fips_npub = crate::identity::fips_npub(&identity_dir)
+            .await
+            .ok()
+            .flatten()
+            .or_else(|| {
+                // Legacy/dev nodes without a seed-derived key fall back
+                // to the upstream daemon's public key on disk.
+                None
+            });
+        let own_fips_npub = match own_fips_npub {
+            Some(n) => Some(n),
+            None => crate::fips::service::read_upstream_npub().await.ok().flatten(),
+        };
+
         let state = federation::build_local_state(
             apps,
             0.0,
@@ -398,6 +415,7 @@ impl RpcHandler {
             tor_active,
             server_name,
             nostr_npub,
+            own_fips_npub,
             &federated_peers,
         );