Add Alire build setup and populate Engine package

- alire.toml rewritten to valid schema ([[depends-on]], real build-switch
  values); restrictions moved to gnat.adc
- gnat.adc: project-wide Jorvik profile, No_Exceptions, No_Implicit_Dynamic_Code,
  SPARK_Mode On
- mafiabot_core.gpr: source dirs, mains, global config pragmas
- Engine: drop the caller-side precondition race on Transition_State; guard moved
  into the protected body with illegal transitions forcing Fault_Halt
- Empty stubs (economy/sockets/exploits): drop Elaborate_Body so they no longer
  compile until implemented

mafiabot_core/src/network/sockets.ads

@@ -1,3 +1,2 @@
-pragma Elaborate_Body;
 package Sockets is
 end Sockets;