proto/discovery: home the recent-request cap and inject request_id from the shell

Two behavior-neutral discovery cleanups:

- Move MAX_RECENT_DISCOVERY_REQUESTS out of the node discovery handler into the
  discovery subsystem limits module and re-export it, keeping the two use sites
  unchanged. Same value and semantics; only its home moves.

- Remove the ambient rand draw from LookupRequest by deleting generate() and
  having the shell draw the random request_id and pass it into the existing
  new() constructor. Same per-request u64 draw, now at the shell, leaving the
  discovery codec free of ambient RNG reads.

src/node/handlers/discovery.rs

@@ -7,13 +7,13 @@
 
 use crate::node::Node;
 use crate::node::reject::DiscoveryReject;
-use crate::proto::discovery::{DiscoveryAction, LookupRequest, LookupResponse};
+use crate::proto::discovery::{
+    DiscoveryAction, LookupRequest, LookupResponse, MAX_RECENT_DISCOVERY_REQUESTS,
+};
 use crate::transport::{TransportAddr, TransportId};
 use crate::{NodeAddr, PeerIdentity};
 use tracing::{debug, info, trace, warn};
 
-const MAX_RECENT_DISCOVERY_REQUESTS: usize = 4096;
-
 /// Shell adapter exposing the live routing tables to the sans-IO discovery
 /// core's `RoutingView` read seam. Lives in `node` so it can read `Node`'s
 /// private `peers` map and call the crate-private tree/bloom predicates.
@@ -483,7 +483,11 @@ impl Node {
 
         let origin = *self.node_addr();
         let origin_coords = self.tree_state().my_coords().clone();
-        let request = LookupRequest::generate(*target, origin, origin_coords, ttl, 0);
+        let request_id = {
+            use rand::RngExt;
+            rand::rng().random()
+        };
+        let request = LookupRequest::new(request_id, *target, origin, origin_coords, ttl, 0);
 
         // Tree-peer bloom-match selection + single encode live in the sans-IO
         // core. The core keeps the tree-only (no non-tree fallback) behavior;