stop trying to interpret TCP, reorder via message counter and hostinfo-creation-order

overlay/batch/batch.go

@@ -1,14 +1,31 @@
 package batch
 
+// SortKey identifies a packet's position in its sender's transmission order.
+// Epoch is a receiver-local ordinal for the tunnel (ConnectionState) that
+// decrypted the packet. A re-handshake replaces the tunnel outright — new
+// hostinfo, new keys, a fresh counter space — and the replacement's epoch is
+// higher, so during the cutover overlap the old tunnel's packets sort first.
+// Counter is the packet's AEAD message counter within that tunnel. The replay
+// window has already rejected duplicates by Commit time, so keys are unique
+// per tunnel and (Epoch, Counter) is a total order with no ties.
+type SortKey struct {
+	Epoch   uint64
+	Counter uint64
+}
+
 type RxBatcher interface {
-	// Commit commits pkt to be flushed by the batch. The caller must keep pkt valid until the next Flush, and not re-use it.
-	Commit(pkt []byte) error
-	// Flush emits every queued packet. The guarantee is per-flow DATA order:
-	// a flow's payload-bearing packets are never reordered relative to each
-	// other. Cross-flow and cross-lane order is not preserved, and two shapes
-	// may legally be overtaken by later same-flow data: pure ACKs (by design,
-	// stale ACKs are ignored) and unparseable shapes such as fragments (an accepted tradeoff; see MultiCoalescer).
-	// Returns the first error observed; keeps draining so one bad packet doesn't hold up the rest.
+	// Commit stages pkt to be flushed by the batch. key must carry the
+	// packet's session epoch and message counter. The caller must keep pkt
+	// valid until the next Flush, and not re-use it.
+	Commit(pkt []byte, key SortKey) error
+	// Flush emits every staged packet. Packets are first sorted by key, so
+	// within each protocol lane emission follows the sender's transmission
+	// order regardless of arrival order. One shape may legally be overtaken
+	// by later same-flow data: a pure TCP ACK, which does not close its
+	// flow's open coalesce chain (a late ACK is just a stale ACK). Cross-lane
+	// order (TCP vs UDP vs everything else) is not preserved.
+	// Returns the first error observed; keeps draining so one bad packet
+	// doesn't hold up the rest.
 	// After Flush returns, committed payload slices may be recycled.
 	Flush() error
 }