roadmap: introspection & observability shipped as RFC 016

Chunks 1-4 all landed; mark the introspection block done (was 'Needs an
RFC'), matching the send_after in-place SHIPPED treatment above it.

ROADMAP.md

@@ -164,10 +164,18 @@ gen_server time layer (RFC 015, shipped above) lands on this, adding only the
 channel-targeting `send_after_to` sibling.
 
 #### Introspection — process_info / get_state / tree dump
-`trace.rs` is the seed. What an actor is parked on, queue depth, stack size; a
+✅ SHIPPED (RFC 016) — runtime introspection & observability, superseding the
-gen_server state snapshot; a supervision-tree walk. The native edge over tokio —
+RFC 000 / 006 / 009 sketches. The mechanism is an internal synchronous read,
-actors already carry pid, name, parent where tokio tasks are anonymous — so it
+not a C ABI: `snapshot()` / `actor_info(pid)` return owned data (pid, names,
-costs little and differentiates a lot. Needs an RFC.
+fine scheduling state, parent edge, trap, monitor/link/joiner counts, mailbox
+depth) carrying `SNAPSHOT_FORMAT_VERSION` (Chunk 1, ps-semantics tearing);
+`tree()` folds that into a parentage forest with orphan re-rooting (Chunk 3).
+Per-actor counters — timeslice overruns, messages-received, and a feature-gated
+approximate time budget — ride hot `AtomicU64` slot fields (Chunk 2). The live
+`observer` gen_server is the read transport over that primitive, behind the
+off-by-default `observer` feature (Chunk 4); it is the read half of the future
+RFC 003 control plane. Wedged-runtime dumps stay gdb's job, and park-reason
+detail / C ABI are explicit non-goals.
 
 #### Worker pool behaviour
 Supervised, interchangeable workers with restart semantics over a shared inbox