net/game: initialise entity swim/walk state from spawn packet and SPLINE_MOVE opcodes

UpdateBlock now stores moveFlags from the LIVING movement block so the
cold-join problem is fixed: entities already swimming or walking when the
client joins get their animation state correctly initialised from the
SMSG_UPDATE_OBJECT CREATE_OBJECT packet rather than waiting for the next
MSG_MOVE_* heartbeat.

Additionally, SMSG_SPLINE_MOVE_START_SWIM, SMSG_SPLINE_MOVE_STOP_SWIM,
SMSG_SPLINE_MOVE_SET_WALK_MODE, SMSG_SPLINE_MOVE_SET_RUN_MODE, and
SMSG_SPLINE_MOVE_SET_FLYING now fire unitMoveFlagsCallback_ with
synthesised flags so explicit server-driven mode transitions update
animation state immediately without waiting for a heartbeat.

src/game/world_packets.cpp

@@ -904,6 +904,7 @@ bool UpdateObjectParser::parseMovementBlock(network::Packet& packet, UpdateBlock
         /*float pitchRate =*/ packet.readFloat();
 
         block.runSpeed = runSpeed;
+        block.moveFlags = moveFlags;
 
         // Spline data
         if (moveFlags & 0x08000000) { // MOVEMENTFLAG_SPLINE_ENABLED