Add Supervisor actor for fault tolerance

[ElFantasma]

Erlang-style supervision:

Restart strategies:

• OneForOne - restart only failed child
• OneForAll - restart all children when any fails
• RestForOne - restart failed child and all started after it

Features:

• Restart intensity tracking (max_restarts in max_seconds)
• Graceful shutdown in reverse order
• Monitors all children for exits

New file: concurrency/src/supervisor.rs

Part of Phase 4 (Supervision).