What is the JDBC URL for Oracle's Data Guard Fast-Start Failover?
You don't setup Fast-Start Failover in the URL.  It's something that you setup on the data source using the setFastConnectionFailoverEnabled(true) function call.

Per here


  An application enables Fast Connection Failover by calling
  setFastConnectionFailoverEnabled(true) on a DataSource instance, before
  retrieving any connections from that instance.
  
  You cannot enable Fast Connection Failover when reinitializing a connection
  cache. You must enable it before using the OracleDataSource instance.


For example

    ods = new OracleDataSource();
    ...
    ods.setFastConnectionFailoverEnabled(true);


Also, it mentions:


  FAST CONNECTION FAILOVER PREREQUISITES
  
  1.) The implicit connection cache is enabled. Fast Connection Failover
  works in conjunction with the JDBC connection caching mechanism.
  This helps applications manage connections to ensure high
  availability.
  
  2.) The application uses service names to connect to the database; the
  application cannot use service IDs (ORACLE_SID). Use a service name
  rather than an SID when setting the OracleDataSource url property.
  
  3.) The underlying database has Release 10 (10.1) or Release 10 (10.2)
  Real Application Clusters (RAC) capability. If failover events are
  not propagated, connection failover cannot occur.
  
  4.) Oracle Notification Service (ONS) is configured and available on the
  node where JDBC is running and on the Oracle RAC. JDBC depends on
  ONS to propagate database events from RAC and notify JDBC of them.
  If ONS is not correctly set up, then implicit connection cache
  creation fails and an ONSException is thrown at the first
  getConnection request.
  
  5.) The JVM in which your JDBC instance is running must have
  oracle.ons.oraclehome set to point to your ORACLE_HOME. For example:
  
  -Doracle.ons.oraclehome=C:\oracle\product\10.2.0\db_1
  
  6.) Set the FastConnectionFailoverEnabled property before making the
  first getConnection request to an OracleDataSource. When Fast
  Connection Failover is enabled, the failover applies to all 
  connections in the connection cache. If your application explicitly
  creates a connection cache using the Connection Cache Manager, then
  you must first set FastConnectionFailoverEnabled before retrieving
     any connections.
  
  7.) The ons.jar must be part of the CLASSPATH for the application.
  The ons.jar can be found in the Oracle Client installation.




Edit:
One last thought, if there ever is a failover, you will need to handle the SQLException:


  ORA-17008: Closed Connection