Add server gameplay transport policy

server/transport_policy.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+from enum import Enum
+
+
+class Delivery(str, Enum):
+    UNRELIABLE_SEQUENCED = "unreliable_sequenced"
+    RELIABLE_ORDERED = "reliable_ordered"
+
+
+_SNAPSHOT_PACKET_TYPES = frozenset({"transform", "npcState"})
+
+
+def delivery_for_packet_type(packet_type: str) -> Delivery:
+    """Return the transport contract for one validated protocol packet type.
+
+    Unknown packet types intentionally default to reliable/ordered. Protocol
+    validation still decides whether an unknown packet is accepted; this
+    function only prevents a future transport adapter from accidentally
+    downgrading control traffic to an unreliable channel.
+    """
+    if packet_type in _SNAPSHOT_PACKET_TYPES:
+        return Delivery.UNRELIABLE_SEQUENCED
+    return Delivery.RELIABLE_ORDERED
+
+
+def is_snapshot_packet(packet_type: str) -> bool:
+    return delivery_for_packet_type(packet_type) is Delivery.UNRELIABLE_SEQUENCED