fix(core): harden exchange send path from full-codebase review

- Cut every fragment of a stream from one encoding (age frozen at the
  first fragment) and bounds-check the slice: re-encoding mid-stream
  corrupted fragments and could panic when the tick went backwards.
- Fold the bloom summary to fit the MTU instead of sending a ~540-byte
  frame that small-MTU links reject; drop the sim's summary MTU
  exemption that hid this.
- Finish a peer's in-flight stream before starting another, and count
  an addressed message as delivered only after its last fragment.
- Alternate custody and broadcast per peer, rotate through addressed
  candidates, and evict the oldest bounce record when full, so neither
  path starves.
- Add Exchange::on_send_error; Node::poll uses it so Busy/Unreachable
  sends are retried rather than silently dropped.
- Clear bounce records per contact, record duplicates against their
  sender, and skip addressed messages that cannot fit the MTU.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tines-core/src/node.rs

@@ -140,10 +140,10 @@ impl<S: Store, T: Transport> Node<S, T> {
     /// Drains all pending transport events, storing any newly received
     /// observations, then emits at most one outbound frame.
     ///
-    /// Returns the number of *new* observations stored this round. A
-    /// [`crate::transport::SendError`] is ignored: the observation stays
-    /// queued only when the transport was `Busy`, otherwise it is dropped and
-    /// re-queued by the next summary round.
+    /// Returns the number of *new* observations stored this round. If the
+    /// transport refuses the frame with `Busy` or `Unreachable`, the exchange
+    /// rolls back and offers the same frame on a later poll; a `TooLarge`
+    /// frame is dropped (see [`Exchange::on_send_error`]).
     pub fn poll(&mut self, now: Tick) -> usize {
         let mut stored = 0usize;
         while let Some(ev) = self.transport.poll() {
@@ -158,7 +158,9 @@ impl<S: Store, T: Transport> Node<S, T> {
             .exchange
             .next_frame(&self.store, mtu, now, &mut self.out)
         {
-            let _ = self.transport.send(&addr, &self.out[..n]);
+            if let Err(err) = self.transport.send(&addr, &self.out[..n]) {
+                self.exchange.on_send_error(&err);
+            }
         }
         stored
     }