Hybrid coordinate warmup: CoordsWarmup message and proactive fallback

Implement hybrid coordinate cache warming strategy: piggyback coords
via CP flag when they fit within transport MTU, send standalone
CoordsWarmup (0x14) message when they don't. On CoordsRequired or
PathBroken receipt, send CoordsWarmup immediately with source-side
rate limiting (default 2s per destination, configurable).

- Add CoordsWarmup = 0x14 session message type (empty body, CP flag)
- Add send_coords_warmup() following send_session_msg() pattern
- Restructure send_session_data() to send standalone warmup before
  data packet when piggybacked coords exceed MTU
- Add immediate CoordsWarmup response in handle_coords_required()
  and handle_path_broken() with per-destination rate limiting
- Add coords_response_interval_ms config (node.session)
- Add RoutingErrorRateLimiter::with_interval() constructor
- Zero transit-path changes: existing try_warm_coord_cache() handles
  CoordsWarmup identically to CP-flagged data packets
- Update design docs (session layer, wire formats, mesh operation,
  configuration)

src/protocol/session.rs

@@ -34,6 +34,8 @@ pub enum SessionMessageType {
     ReceiverReport = 0x12,
     /// Path MTU notification (discovered path MTU).
     PathMtuNotification = 0x13,
+    /// Standalone coordinate cache warming (empty body, coords in CP flag).
+    CoordsWarmup = 0x14,
 
     // Link-layer error signals (0x20-0x2F) — plaintext, from transit routers
     /// Router cache miss — needs coordinates (link-layer error signal).
@@ -52,6 +54,7 @@ impl SessionMessageType {
             0x11 => Some(SessionMessageType::SenderReport),
             0x12 => Some(SessionMessageType::ReceiverReport),
             0x13 => Some(SessionMessageType::PathMtuNotification),
+            0x14 => Some(SessionMessageType::CoordsWarmup),
             0x20 => Some(SessionMessageType::CoordsRequired),
             0x21 => Some(SessionMessageType::PathBroken),
             _ => None,
@@ -73,6 +76,7 @@ impl fmt::Display for SessionMessageType {
             SessionMessageType::SenderReport => "SenderReport",
             SessionMessageType::ReceiverReport => "ReceiverReport",
             SessionMessageType::PathMtuNotification => "PathMtuNotification",
+            SessionMessageType::CoordsWarmup => "CoordsWarmup",
             SessionMessageType::CoordsRequired => "CoordsRequired",
             SessionMessageType::PathBroken => "PathBroken",
         };