docs: README streaming/SSE sections, ROADMAP v0.3 done, crud SSE ticker (v0.3 chunk 4)

README: write_timeout in the Config sample, read/write timeout-split
semantics spelled out, new Streaming Responses and Server-Sent Events
sections (producer contract, 1.0 fallback, heartbeat liveness), roadmap
summary bumped. ROADMAP: v0.3 marked done with the as-landed decisions
(pull shape per the user's fork answer; Q3 timeout split as proposed)
and verified exit criteria. crud example: GET /ticker SSE route — one
event/second, producer exits on SseClosed or the example's shutdown
flag (so an open ticker doesn't block AllDone past the drain).

examples/crud.rs

@@ -204,6 +204,28 @@ fn parse_id(s: &str) -> Option<u64> {
 // Handlers
 // ---------------------------------------------------------------------------
 
+/// SSE demo: `curl -N localhost:8080/ticker` streams a tick every second
+/// (with `: keep-alive` comments if it ever goes quiet). The producer
+/// exits on SseClosed (client gone / write timeout / shutdown drain) or
+/// when the example is shutting down.
+fn ticker(conn: Conn, _next: Next) -> Conn {
+    let (conn, events) = conn.sse();
+    smarm::spawn(move || {
+        let mut n: u64 = 0;
+        loop {
+            if SHUTTING_DOWN.load(std::sync::atomic::Ordering::Relaxed) {
+                return; // dropping `events` ends the stream cleanly
+            }
+            if events.send("tick", &n.to_string()).is_err() {
+                return; // SseClosed
+            }
+            n += 1;
+            smarm::sleep(std::time::Duration::from_secs(1));
+        }
+    });
+    conn
+}
+
 fn list(conn: Conn, _next: Next) -> Conn {
     let (tx, rx) = channel::<(u16, Vec<u8>)>();
     store().send(Request::List { reply: tx }).ok();
@@ -278,7 +300,8 @@ fn main() {
                 .post(  "/users",     create)
                 .get(   "/users/:id", get_one)
                 .put(   "/users/:id", update)
-                .delete("/users/:id", delete),
+                .delete("/users/:id", delete)
+                .get(   "/ticker",    ticker),
         );
 
     let cfg = Config::new("127.0.0.1:8080".parse().unwrap());