RFC 016 Chunk 4: observer example with ps-style + tree dump

A runnable examples/observer.rs (required-features = ["observer"]) that
stands up a named service + two parked workers, starts the observer, and
renders a snapshot as a ps-style table and the parentage forest indented.
The observer appears in its own dump, caught running while it serves the
snapshot call — transport over the same read every consumer sees.

Complements the runnable doctest already on observer::start.

Cargo.toml

@@ -75,3 +75,8 @@ harness = false
 [[bench]]
 name = "switch_cost"
 harness = false
+
+# RFC 016 Chunk 4 — the live observer dump. Needs the optional gen_server.
+[[example]]
+name = "observer"
+required-features = ["observer"]