Add Pid (Process Identity) type

Add unique process identifiers:

```rust
pub struct Pid(u64);

pub enum ExitReason {
    Normal,
    Error(String),
    Shutdown,
    Linked(Pid),
}
```

Features:
- Globally unique across process lifetime
- Lightweight (u64-based)
- Foundation for distributed support

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

Part of Phase 3 (Process Primitives).