Merge refactor-sans-io: disambiguate the "discovery" name

Forward-merge the mesh-lookup "discovery" disambiguation onto the next
line: the lookup handler module and Node.lookup field renames, the lookup
metric types, the dual-emitted lookup/discovery metric family, and the
node.discovery -> node.lookup/node.rendezvous config split with its
deprecation shim.

Conflicts resolved by keeping next's XX handshake path (the inbound
responder stores pending_inbound for msg3 rather than promoting on msg1)
and renaming its reset_discovery_backoff call sites to reset_lookup_backoff;
the CHANGELOG Unreleased entries from both lines are combined.

src/node/stats.rs

@@ -238,7 +238,7 @@ pub struct ForwardingStatsSnapshot {
 }
 
 #[derive(Clone, Debug, Default, Serialize)]
-pub struct DiscoveryStatsSnapshot {
+pub struct LookupStatsSnapshot {
     pub req_received: u64,
     pub req_decode_error: u64,
     pub req_duplicate: u64,