REPL fixes + an nREPL server for editor-connected dev

The line REPL was broken (read-line called nil — the __stdin-read-line host seam
the clojure.core *in* reader drives was never implemented on Chez) and didn't
load the project, so (require '[some.lib]) failed. Now:

- __stdin-read-line reads a line from stdin (get-line); read-line / read / the
  REPL work.
- repl resolves the project first (deps on the roots, native libs loaded), so
  libraries are available — same context a run gets.
- jolt.nrepl: a jolt-native nREPL server (bencode over a loopback jolt.ffi
  socket) speaking the real protocol — clone / describe / eval / load-file /
  close, with stdout capture, :ns-scoped eval (in-ns; binding *ns* doesn't drive
  load-string resolution here), and real error text. 'joltc nrepl [port]' applies
  the project then serves; writes .nrepl-port. Editors (CIDER/Calva/Cursive)
  connect and develop live; project libraries load in the session.
- ex-message returns nil for raw Chez conditions, so jolt.host/condition-message
  exposes the condition text; the REPL and nREPL surface it instead of an opaque
  #<compound condition>.

Why native, not real nREPL: nrepl.server is welded to java.util.concurrent
executors, two compiled Java helper classes, a DynamicClassLoader, Compiler
internals and a JVMTI agent — not faithfully shimmable. The wire protocol, which
is what clients depend on, is small and implemented directly.

Runtime .ss + jolt-core, no re-mint. Full gate green.

docs/building-and-deps.md

@@ -61,7 +61,8 @@ bin/joltc run -m NS [args]  # resolve deps.edn, load NS, call its -main
 bin/joltc run FILE          # resolve deps.edn, load a Clojure file
 bin/joltc -M:alias [args]   # run the alias's :main-opts
 bin/joltc -A:alias [args]   # add the alias's paths/deps, then run the rest
-bin/joltc repl              # start a line REPL
+bin/joltc repl              # start a line REPL (project deps + native libs loaded)
+bin/joltc nrepl [port]      # start an nREPL server (default 7888) for editors
 bin/joltc path              # print the resolved source roots (':'-joined)
 bin/joltc <task>            # run a deps.edn :tasks entry
 ```