RFC 016 Chunk 4: observer gen_server (feature-gated)

A thin GenServer consumer of the Chunk-1 read primitive — the live
observer process (D12). ObserverRequest/ObserverReply are the wire
contract (D11); the version rides along on the snapshot/tree payloads,
which already carry SNAPSHOT_FORMAT_VERSION (D1). Behind the new
`observer` Cargo feature, off by default (D10): the primitive stays
always-on, only the transport is gated. Cast is Infallible, so the
server takes no async traffic and handle_cast is statically unreachable.

Gated integration test proves each verb relays exactly what the
corresponding primitive returns (snapshot/tree/actor_info), incl. a
forged-pid None and a live Parked classification.

src/lib.rs

@@ -28,6 +28,8 @@ pub mod pg;
 pub mod link;
 pub mod gen_server;
 pub mod introspect;
+#[cfg(feature = "observer")]
+pub mod observer;
 pub mod runtime;
 pub(crate) mod raw_mutex;
 pub(crate) mod slot_state;
@@ -59,6 +61,8 @@ pub use introspect::{
     actor_info, snapshot, tree, tree_from, ActorInfo, ActorState, RuntimeSnapshot, RuntimeTree,
     TreeNode, SNAPSHOT_FORMAT_VERSION,
 };
+#[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 mutex::{LockTimeout, Mutex, MutexGuard};