feat: add Supervisor and DynamicSupervisor

Implements Erlang/OTP-style supervision for fault-tolerant systems:

Supervisor:
- Manages a static set of child processes defined at start
- Monitors children and restarts them according to restart strategy
- Supports restart strategies: OneForOne, OneForAll, RestForOne
- Child specs with: restart type, shutdown behavior, child type

DynamicSupervisor:
- Manages dynamically spawned children
- Supports start_child/terminate_child for runtime management
- Configurable max_children limit

Features:
- Automatic restart on child crash (permanent, temporary, transient)
- Max restart intensity to prevent restart loops
- Graceful shutdown with configurable timeout
- Type-safe child handle abstraction
- Full integration with GenServer Backend system
- Comprehensive test suite (unit + integration tests)

Author: unbalancedparentheses

concurrency/src/lib.rs

@@ -9,6 +9,7 @@ mod process;
 pub mod process_table;
 pub mod registry;
 mod stream;
+pub mod supervisor;
 mod time;
 
 #[cfg(test)]
@@ -27,4 +28,10 @@ pub use process::{send, Process, ProcessInfo};
 pub use process_table::LinkError;
 pub use registry::RegistryError;
 pub use stream::spawn_listener;
+pub use supervisor::{
+    BoxedChildHandle, ChildHandle, ChildInfo, ChildSpec, ChildType, DynamicSupervisor,
+    DynamicSupervisorCall, DynamicSupervisorCast, DynamicSupervisorError, DynamicSupervisorResponse,
+    DynamicSupervisorSpec, RestartStrategy, RestartType, Shutdown, Supervisor, SupervisorCall,
+    SupervisorCast, SupervisorCounts, SupervisorError, SupervisorResponse, SupervisorSpec,
+};
 pub use time::{send_after, send_interval};