proto: rename the mesh discovery subsystem module from discovery to lookup

Rename src/proto/discovery to src/proto/lookup and bring the module's naming
onto the lookup stem, matching its concept: a mesh lookup of a node's
coordinates from its pubkey, sent into the mesh as a bloom-filter-guided
multicast request that returns a unicast response with the coordinates.

- the module directory and its declaration (proto::discovery -> proto::lookup)
- exported types: DiscoveryAction -> LookupAction, DiscoveryBackoff ->
  LookupBackoff, DiscoveryForwardRateLimiter -> LookupForwardRateLimiter,
  MAX_RECENT_DISCOVERY_REQUESTS -> MAX_RECENT_LOOKUP_REQUESTS, and the Discovery
  state struct -> Lookup
- internal terminology: doc comments, the "overlay-lookup" phrasing, the
  empty_discovery/suppressing_discovery test helpers, and the disc
  parameter/variable all take the lookup names
- the already-lookup-named wire types (LookupRequest/LookupResponse) are unchanged

Behavior-neutral: no wire bytes or decision logic change. Two references are
intentionally kept as "discovery": the still-named node::handlers::discovery
shell module and the node.discovery.* config keys, which belong to the broader
disambiguation of the shell, config, and metric surfaces still to come.

src/node/mod.rs

@@ -43,9 +43,9 @@ use crate::cache::CoordCache;
 use crate::node::session::SessionEntry;
 use crate::peer::{ActivePeer, PeerConnection};
 use crate::proto::bloom::{BloomFilter, BloomState};
-use crate::proto::discovery::{Discovery, DiscoveryBackoff, DiscoveryForwardRateLimiter};
 use crate::proto::fmp::Fmp;
 use crate::proto::fsp::Fsp;
+use crate::proto::lookup::{Lookup, LookupBackoff, LookupForwardRateLimiter};
 use crate::proto::mmp::Mmp;
 use crate::proto::routing::{self, Router, RoutingErrorRateLimiter};
 use crate::proto::stp::TreeState;
@@ -337,7 +337,7 @@ pub struct Node {
     // === Discovery ===
     /// Discovery-subsystem state: recent-request dedup cache, in-flight
     /// lookups, originator-side backoff, and transit-side forward limiter.
-    discovery: Discovery,
+    discovery: Lookup,
 
     // === Counters ===
     /// Next link ID to allocate.
@@ -678,9 +678,9 @@ impl Node {
             coords_response_rate_limiter: RoutingErrorRateLimiter::with_interval_ms(
                 coords_response_interval_ms,
             ),
-            discovery: Discovery::new(
-                DiscoveryBackoff::with_params(backoff_base_secs, backoff_max_secs),
-                DiscoveryForwardRateLimiter::with_interval_ms(forward_min_interval_secs * 1000),
+            discovery: Lookup::new(
+                LookupBackoff::with_params(backoff_base_secs, backoff_max_secs),
+                LookupForwardRateLimiter::with_interval_ms(forward_min_interval_secs * 1000),
             ),
             pending_connects: Vec::new(),
             retry_pending: HashMap::new(),
@@ -843,7 +843,7 @@ impl Node {
             coords_response_rate_limiter: RoutingErrorRateLimiter::with_interval_ms(
                 coords_response_interval_ms,
             ),
-            discovery: Discovery::new(DiscoveryBackoff::new(), DiscoveryForwardRateLimiter::new()),
+            discovery: Lookup::new(LookupBackoff::new(), LookupForwardRateLimiter::new()),
             pending_connects: Vec::new(),
             retry_pending: HashMap::new(),
             nostr_discovery: None,
@@ -2541,7 +2541,7 @@ impl Node {
     /// Iterate over pending discovery lookups for diagnostics.
     pub fn pending_lookups_iter(
         &self,
-    ) -> impl Iterator<Item = (&NodeAddr, &crate::proto::discovery::PendingLookup)> {
+    ) -> impl Iterator<Item = (&NodeAddr, &crate::proto::lookup::PendingLookup)> {
         self.discovery.pending_lookups.iter()
     }