nREPL: make the built-in server middleware-extensible

The built-in nREPL stays minimal (clone/describe/eval/load-file/close) but now
composes a middleware stack so a library can add the heavier features (sessions,
interruptible-eval, completion, lookup) without bloating core.

- A middleware is (fn [handler] (fn [request] ...)); request carries :reply (a
  thread-safe send that adds id/session) plus the wire fields. List them in
  deps.edn :nrepl/middleware (symbols -> a middleware fn or a vector of them);
  jolt.nrepl composes them over the built-in handler.
- Public seam: respond, evaluate, register-ops!, new-session, err-msg.
- Per-connection send lock so middleware replying from other threads don't
  interleave bytes. describe advertises built-in + registered ops.

deps.clj surfaces :nrepl/middleware; jolt.main passes it to the server. Built-in
behavior unchanged when no middleware is declared. Runtime, no re-mint.

jolt-core/jolt/deps.clj

@@ -124,7 +124,10 @@
      {:roots (vec (distinct (concat project-roots dep-roots)))
       :main-opts main-opts
       :tasks (:tasks edn)
-      :natives (vec (distinct (concat (:jolt/native edn) dep-natives)))})))
+      :natives (vec (distinct (concat (:jolt/native edn) dep-natives)))
+      ;; nREPL middleware a library contributes (jolt.nrepl composes them over its
+      ;; built-in handler) — symbols resolving to a middleware fn or a vector of them.
+      :nrepl-middleware (:nrepl/middleware edn)})))
 
 (defn has-deps-edn? [project-dir]
   (file-exists? (str project-dir "/deps.edn")))