feat(pubsub): topic table gen_server — v0.5 chunk 1

urus::pubsub, phoenix_pubsub-shaped, local node only. Independent of
HTTP (imports smarm only); crate-extraction candidate.

Design as ratified (handoff Q1-Q5, user said "push" on the leans):
- Q1 generic: PubSub<M> per instance, zero-cost, no downcasts.
  Heterogeneous events = app-side enum M. v0.6 channels build on it.
- Q2 Receiver: subscribe(topic) -> Receiver<Arc<M>>, fresh channel per
  subscription, subscriber owns its loop. Fan-in via a Sender-passing
  subscribe_with is a compatible later addition, deliberately not v1.
  subscribe_as(pid, topic) pins the subscription lifetime to an
  explicit pid for relay patterns (session actor subscribes, spawned
  relay receives) — without it the monitor would watch the wrong actor.
- Q3 unbounded: broadcast never blocks the table (bounded-block =
  head-of-line across ALL topics; bounded-drop = silent loss). Memory
  risk documented; TWO cleanup paths: monitor Down on subscriber death
  (eager) + prune-on-send-failure for dropped receivers (lazy, on next
  broadcast). Bench before sharding.
- Q4 user-owned ServerRef wrapper. DISCOVERY: the ratified optional
  register-by-name helper is NOT implementable against smarm's
  registry — it maps name -> Pid, and a Pid cannot be turned back into
  a ServerRef (the ref is the inbox sender). Needs smarm support or a
  global type-erased map; deferred, noted in module docs. pid()
  exposed for introspection.
- Q5 unique per (pid, topic): HashMap<Topic, HashMap<Pid, Sender>>,
  subscribe idempotent (replaces sender; stale receiver's channel
  closes). One monitor per live pid (monitored: HashSet<Pid>, retired
  in handle_down so slot-reuse pids re-monitor). handle_down does the
  full-scan cleanup as ratified; pid->topics reverse index is the
  documented optimization if deaths ever measure hot.

Subscribe is a call (table updated before return: a broadcast issued
right after by the same caller is seen); unsubscribe/broadcast are
casts. subscriber_count(topic) added as a call — needed by the tests
to observe async cleanup, legitimate API anyway.

8 runtime-backed unit tests (smarm::run + collect-outside-assert-after
pattern from smarm's own gen_server tests), including: Arc payload
ptr-equality across subscribers, monitor-path pruning with NO
broadcast issued (isolates it from the lazy path), broadcast_from
self-skip, resubscribe replacement.

Suite: 67 unit + 40 integration + 2 doc, green. smarm PRISTINE.

src/lib.rs

@@ -28,12 +28,14 @@ pub mod conn_registry;
 pub mod serve;
 pub mod sse;
 pub mod ws;
+pub mod pubsub;
 
 // Re-exports — what most users want at the crate root.
 pub use conn::{Assigns, Body, Conn, HeaderMap, HttpVersion, Method, Params, RespBody, StreamBody};
 pub use plug::{Next, Pipeline, Plug};
 pub use router::Router;
 pub use sse::{EventSender, SseClosed};
+pub use pubsub::{PubSub, PubSubDown};
 pub use ws::{Message, WsClosed, WsHandler, WsSender};
 pub use serve::{
     serve, serve_with, serve_with_shutdown, shutdown_handle, Config, Handle, ShutdownSignal,