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.

Cargo.toml

@@ -33,6 +33,11 @@ budget-accounting = []
 # and unflagged; only the optional gen_server transport sits behind this, so a
 # release build pays nothing for an observer it never starts.
 observer = []
+# RFC 010 c1: clustering. Off by default — the default build stays libc-only,
+# byte-for-byte (gate checked per phase). serde is the payload contract,
+# postcard the payload codec; both minimal (no default features). Everything
+# cluster-shaped lives behind this flag.
+cluster = ["dep:serde", "dep:postcard"]
 # Run-queue selection: exactly one, compile-time (see src/run_queue.rs).
 # Non-default variants need --no-default-features (features are additive).
 rq-mutex = []
@@ -44,6 +49,9 @@ cc = "1"
 
 [dependencies]
 libc = "0.2"
+# RFC 010 §2 — only compiled under `--features cluster`.
+serde = { version = "1", default-features = false, optional = true }
+postcard = { version = "1", default-features = false, optional = true }
 
 [target.'cfg(loom)'.dependencies]
 loom = "0.7"