feat(cluster): RFC 010 c1 — cluster feature flag + optional serde/postcard deps

Off by default; the default build stays libc-only. serde (payload contract)
and postcard (payload codec) are optional, no default features. src/cluster.rs
is an intentionally empty cfg-gated stub so the flag's default-build
invariance is reviewable in isolation.

src/lib.rs

@@ -14,6 +14,8 @@
 pub mod actor;
 pub mod causal;
 pub mod channel;
+#[cfg(feature = "cluster")]
+pub mod cluster;
 pub mod context;
 pub mod gen_server;
 pub mod gen_statem;