feat(cluster): RFC 010 c8 — exposure registry + fixed-seed type hashing

Nothing local is remotely reachable by default (RFC §4). expose(Name<M>)
marks a name remotely addressable and registers M's decoder under
type_hash::<M>(); expose_type::<M>() registers only the decoder (the
reply-to path). exposed_names() is the auditable remote surface.

D3's watchable fold, resolved against the code as it stands (stated in the
module docs): register() ALREADY stamps every named holder watchable ('no
successfully-registered actor can die unflagged', registry.rs), so an
exposed name's holder needs no extra mark — and re-registration after a
holder's death re-stamps the new holder for free, which a per-tenancy mark
taken at expose time could not do. The cluster's own mark_watchable
set-site is therefore the pid crossing the wire (frame serialization, c10)
— the exact analog of the membrane crossing. c8 adds only the name/type
state neither the registry nor slot bits can carry. No new pid registry;
RFC §4 honored.

One-viable calls, flagged:
- State lives on RuntimeInner (the pg pattern: leaf RawMutex field,
  cfg-gated behind cluster, zero-cost-when-off per c1) — c9's inbound
  decode consults it per frame; manager-held state would serialize every
  remote delivery through one gen_server.
- type_hash = FNV-1a 64 (fixed seed: the offset basis) over TypeId: a
  constant of the binary — stable across runs of the same build (the scope
  the build-hash handshake reduces the mesh to), deliberately not across
  builds. Collisions degrade to decode error / refused channel, never a
  misroute (the NoChannel guarantee, RFC §3).
- Decoder = decode-and-deliver-to-pid Arc closure capturing M (the one
  typed site): decode_payload then send_dyn. Wire-name → pid resolution
  stays OUTSIDE — that is c9's single seam, which calls decode_deliver.
  Arc so the call happens with the exposure lock RELEASED: send_dyn takes
  the registry lock, a mutual Leaf (the runtime asserts on nesting — caught
  live by the first test run).
- expose is a name-level fact, valid for an unregistered name (names
  late-bind; c9 resolves per delivery).

tests/cluster_expose.rs 5/0 stable x5, purely local per roadmap:
exposed/unexposed lookup + audit listing; decoder registration and the
delivery contract (happy path into a registered String channel; unknown
hash; corrupt bytes; wrong channel refused — never misrouted); distinct
types distinct hashes; expose/bridge-crossing agreement via the shared
watchable observable (terminal_reason after holder death); hash stability
across runs in the same binary via a c4-harness re-exec. Payload types are
std types — the crate's serde is derive-less by design, user crates bring
their own derive.

All cluster suites regression-clean (envelope 15, handshake 11, transport
11, lifecycle 1, liveness 3, connect 9, two_node 3, membership 4, mesh 2);
clippy --lib green both configs; fmt clean; default build compiles.

src/cluster.rs

@@ -13,6 +13,7 @@ pub mod connect;
 pub mod connector;
 pub mod discovery;
 pub mod envelope;
+pub mod expose;
 pub mod handshake;
 pub mod manager;
 pub mod membership;
@@ -36,6 +37,7 @@ pub use conn::{spawn_established, ConnHandle};
 pub use connect::{dial, spawn_acceptor, AcceptorHandle};
 pub use connector::{spawn_connector, ConnectorHandle};
 pub use discovery::{Discovery, StaticSeeds, Strategy};
+pub use expose::{expose, expose_type, type_hash, DeliverError};
 pub use manager::{Manager, MANAGER};
 pub use membership::{subscribe, view, MembershipEvents, NodeEvent, NodeInfo};