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/proto/discovery/limits.rs

@@ -16,6 +16,14 @@
 use crate::NodeAddr;
 use alloc::collections::BTreeMap;
 
+// ============================================================================
+// Receive-side: Request dedup cache bound
+// ============================================================================
+
+/// Maximum number of recent LookupRequests retained for dedup and
+/// reverse-path routing before the cache is treated as full.
+pub(crate) const MAX_RECENT_DISCOVERY_REQUESTS: usize = 4096;
+
 // ============================================================================
 // Originator-side: Discovery Backoff
 // ============================================================================

src/proto/discovery/mod.rs

@@ -26,7 +26,9 @@ pub(crate) use core::{
     initiate_gate, on_response_accepted, plan_forward, plan_initiate, plan_response_route,
     poll_pending,
 };
-pub(crate) use limits::{DiscoveryBackoff, DiscoveryForwardRateLimiter};
+pub(crate) use limits::{
+    DiscoveryBackoff, DiscoveryForwardRateLimiter, MAX_RECENT_DISCOVERY_REQUESTS,
+};
 #[cfg(test)]
 pub(crate) use state::RecentRequest;
 pub(crate) use state::{Discovery, PendingLookup};

src/proto/discovery/tests/wire.rs

@@ -36,8 +36,9 @@ fn test_lookup_request_generate() {
     let origin = make_node_addr(2);
     let coords = make_coords(&[2, 0]);
 
-    let req1 = LookupRequest::generate(target, origin, coords.clone(), 5, 0);
-    let req2 = LookupRequest::generate(target, origin, coords, 5, 0);
+    use rand::RngExt;
+    let req1 = LookupRequest::new(rand::rng().random(), target, origin, coords.clone(), 5, 0);
+    let req2 = LookupRequest::new(rand::rng().random(), target, origin, coords, 5, 0);
 
     // Random IDs should differ
     assert_ne!(req1.request_id, req2.request_id);

src/proto/discovery/wire.rs

@@ -48,19 +48,6 @@ impl LookupRequest {
         }
     }
 
-    /// Generate a new request with a random ID.
-    pub fn generate(
-        target: NodeAddr,
-        origin: NodeAddr,
-        origin_coords: TreeCoordinate,
-        ttl: u8,
-        min_mtu: u16,
-    ) -> Self {
-        use rand::RngExt;
-        let request_id = rand::rng().random();
-        Self::new(request_id, target, origin, origin_coords, ttl, min_mtu)
-    }
-
     /// Decrement TTL for forwarding.
     ///
     /// Returns false if TTL was already 0.