Add MTU fields to lookup packets for path MTU discovery

Add min_mtu (u16) to LookupRequest and path_mtu (u16) to
LookupResponse, enabling the discovery system to report transport
MTU capability along the lookup path.

LookupRequest carries min_mtu (origin's minimum MTU requirement,
default 0 = no requirement). LookupResponse carries path_mtu
(initialized to u16::MAX by the target, reduced by transit nodes
via min(path_mtu, outgoing_link_mtu) on the reverse path).

path_mtu is a transit annotation like SessionDatagram.path_mtu and
is NOT included in the proof signature. The originator stores the
discovered path_mtu in CacheEntry alongside cached coordinates.

Wire format: +2 bytes each for LookupRequest and LookupResponse.

src/cache/entry.rs

@@ -13,6 +13,12 @@ pub struct CacheEntry {
     last_used: u64,
     /// When this entry expires (Unix milliseconds).
     expires_at: u64,
+    /// Path MTU discovered during lookup (if available).
+    ///
+    /// Set from the `LookupResponse.path_mtu` field when a discovery
+    /// response is cached. `None` when populated from SessionSetup or
+    /// other sources that don't carry path MTU information.
+    path_mtu: Option<u16>,
 }
 
 impl CacheEntry {
@@ -23,6 +29,7 @@ impl CacheEntry {
             created_at: current_time_ms,
             last_used: current_time_ms,
             expires_at: current_time_ms.saturating_add(ttl_ms),
+            path_mtu: None,
         }
     }
 
@@ -46,6 +53,16 @@ impl CacheEntry {
         self.expires_at
     }
 
+    /// Get the path MTU discovered during lookup, if available.
+    pub fn path_mtu(&self) -> Option<u16> {
+        self.path_mtu
+    }
+
+    /// Set the path MTU discovered during lookup.
+    pub fn set_path_mtu(&mut self, mtu: u16) {
+        self.path_mtu = Some(mtu);
+    }
+
     /// Check if this entry has expired.
     pub fn is_expired(&self, current_time_ms: u64) -> bool {
         current_time_ms > self.expires_at