pg: Phase 1 — identity & storage substrate (RFC 012)

Add a process-groups module (name -> multiset<Member>) sibling to the
registry, plus the cluster-shaped identity it is keyed on.

- NodeId/Incarnation u32 newtypes; Member { node, incarnation, pid } at
  final (BEAM NEW_PID_EXT-shaped) layout.
- Config::node_id/incarnation builder setters beside wake_slot, defaulting
  to a fixed single-node value; threaded through RuntimeInner::new.
- process_groups: RawMutex<ProcessGroups> on RuntimeInner, Leaf class,
  mirroring the registry field.
- remove_where: the one dumb predicate-eviction primitive (no liveness wired
  yet; first caller is the Phase 2 death hook).
- Public surface (join/leave/members/pick) in pre-liveness/pre-monitor form,
  Pid-shaped; node/incarnation filled from runtime identity.
- Structural unit tests on synthetic members: idempotent join, multiset
  semantics across groups, generation-distinct members, leave + empty-group
  pruning, predicate sweep, incarnation-sweep shape.

src/lib.rs

@@ -24,6 +24,7 @@ pub mod io;
 pub mod mutex;
 pub mod monitor;
 pub mod registry;
+pub mod pg;
 pub mod link;
 pub mod gen_server;
 pub mod runtime;
@@ -54,6 +55,7 @@ pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
 pub use mutex::{LockTimeout, Mutex, MutexGuard};
 pub use pid::Pid;
+pub use pg::{join, leave, members, pick, Incarnation, Member, NodeId};
 pub use registry::{name_of, register, unregister, whereis, RegisterError};
 pub use runtime::{init, Config, Runtime};
 pub use scheduler::{