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.

Cargo.toml

@@ -15,6 +15,11 @@ smarm-trace = []
 # (D6). The `ActorInfo.budget_cycles` field exists regardless; it just stays 0
 # unless this is enabled.
 budget-accounting = []
+# RFC 016 Chunk 4: the live observer gen_server (src/observer.rs). Off by
+# default (DECISION D10) — the read primitive (Chunks 1–3) is always present
+# and unflagged; only the optional gen_server transport sits behind this, so a
+# release build pays nothing for an observer it never starts.
+observer = []
 # Run-queue selection: exactly one, compile-time (see src/run_queue.rs).
 # Non-default variants need --no-default-features (features are additive).
 rq-mutex = []