monitor/registry: terminal-outcome record — a raced watch can recover the real down reason (soak sig 4)

A watch installed after its target's death has, until now, only NoProc to
report — but the bridge's proxies install their native watch asynchronously
after acquire returns, so a link established before a crash (from the BEAM's
view) could still lose the panic's translated reason to that blanket NoProc
(width-20 soak signature 4: link_test.exs:26, 1/600 full-suite, 3/2000
link-only, all whereis-miss; deterministic repro in the bridge suite).

Two primitives, no change to monitor()'s own Erlang-faithful stale-pid
semantics — the upgrade is the caller's deliberate act:

- finalize_actor stamps the slot with (generation, DownReason) under the same
  cold-lock block that publishes the outcome. The record survives reclaim,
  registry pruning, and the next tenant's install; only the slot's next death
  overwrites it. terminal_reason(pid) reads it generation-matched.
- resolve_name(name) is whereis with the corpse kept: the dead-holder arm
  returns the stored pid it prunes (NameResolution::Corpse) instead of
  discarding the only evidence of who died — whereis itself prunes on the way
  out, so a whereis-then-lookup consumer would find the evidence already
  destroyed. Live/Unbound match whereis's Some/None; the name heals exactly
  as before.

Contract pinned in tests/terminal_outcome_after_death.rs: one record per way
of dying (Exit/Panic/Stopped), no record while live, corpse capture + heal on
resolve_name, record independence from registry pruning, survival across slot
re-tenancy, overwrite at the next tenancy's death.

src/lib.rs

@@ -72,13 +72,13 @@ pub use introspect::{StackInfo,
 #[cfg(feature = "observer")]
 pub use observer::{ObserverReply, ObserverRequest};
 pub use link::{link, trap_exit, unlink, ExitSignal};
-pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
+pub use monitor::{demonitor, monitor, terminal_reason, Down, DownReason, Monitor, MonitorId};
 pub use mutex::{LockTimeout, Mutex, MutexGuard};
 pub use pid::{Addressable, Erased, Name, Pid, RawPid};
 pub use pg::{dispatch, join, leave, members, members_as, pick, pick_as, Incarnation, Member, NodeId};
 pub use registry::{
-    install, lookup_as, register, send, send_dyn, send_to, unregister, whereis, RegisterError,
-    SendError,
+    install, lookup_as, register, resolve_name, send, send_dyn, send_to, unregister, whereis,
+    NameResolution, RegisterError, SendError,
 };
 pub use runtime::{init, Config, Runtime};
 pub use scheduler::{