Document latest-only retention for newly attached QUIC peers

One sentence each in the C# QUIC README and the transport-planes doc: a
provider retains the newest latest-only frame per (channel, body) and
delivers it to a peer as soon as it attaches, bounded by the number of
distinct keys.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/GameCult.Mesh.Quic/README.md

@@ -66,7 +66,8 @@ var frame = await session.ReceiveAsync(cancellationToken);
 - `ReliableOrdered` uses one ordered QUIC stream.
 - `LatestOnly` uses independent streams and keeps at most one pending frame per
   channel and body. A newer `(producer epoch, sequence)` generation replaces
-  queued state.
+  queued state. The provider also retains the newest `LatestOnly` frame per
+  `(channel, body)` and delivers it to each client as soon as it attaches.
 - `Unreliable` fails closed in this adapter because `System.Net.Quic` does not
   expose QUIC datagrams. Use a native MsQuic connector when datagram semantics
   are required.

src/GameCult.Mesh/docs/transport-planes.md

@@ -55,7 +55,11 @@ broadcast enqueues into a keyed, coalescing outbox owned by each physical peer
 and returns without waiting for network delivery. The peer's transport worker
 owns stream writes and eviction after disconnect or send failure. Thus a slow,
 stalled, or departed client cannot backpressure a simulation tick, while a
-healthy peer still converges on the newest generation. Reliable-ordered
+healthy peer still converges on the newest generation. The provider also
+retains the newest latest-only frame per `(channel, body)` and seeds it into
+each newly attached client, bounded by the number of distinct keys, so a peer
+that connects or reconnects after a broadcast still converges on current
+state without a service-side replay loop. Reliable-ordered
 broadcast remains completion-bearing because dropping or replacing it would
 violate its declared delivery contract; caller cancellation can stop that
 operation.