Add DynamicSupervisor for runtime child creation

For dynamically created children (OneForOne strategy only):

```rust
pub enum DynSupMessage {
    StartChild(ChildSpec),
    TerminateChild(Pid),
    WhichChildren,
    CountChildren,
}
```

Features:
- Optional `max_children` limit
- Optional registration by name

**New file:** `concurrency/src/dynamic_supervisor.rs`

Part of Phase 4 (Supervision).