Phase 8: REPL, printing, source maps, and nREPL server

- IPrintWithWriter protocol + CljElixir.Printer module with pr-str,
  print-str, pr, prn for all BEAM types (EDN-like output)
- Source-mapped error messages: line/col metadata from reader now
  propagated through transformer into Elixir AST for accurate error
  locations in .clje files
- Interactive REPL (mix clje.repl) with multi-line input detection,
  history, bindings persistence, and pr-str formatted output
- nREPL server (mix clje.nrepl) with TCP transport, Bencode wire
  protocol, session management, and core operations (clone, close,
  eval, describe, ls-sessions, load-file, interrupt, completions).
  Writes .nrepl-port for editor auto-discovery.

92 new tests (699 total, 0 failures).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/clj_elixir/repl.ex

@@ -0,0 +1,110 @@
+defmodule CljElixir.REPL do
+  @moduledoc """
+  CljElixir Read-Eval-Print Loop engine.
+
+  Maintains state across evaluations: bindings persist,
+  modules defined in one evaluation are available in the next.
+  """
+
+  defstruct bindings: [],
+            history: [],
+            counter: 1,
+            env: nil
+
+  @doc "Create a new REPL state"
+  def new do
+    %__MODULE__{}
+  end
+
+  @doc """
+  Evaluate a CljElixir source string in the given REPL state.
+  Returns {:ok, result_string, new_state} or {:error, error_string, new_state}.
+  """
+  def eval(source, state) do
+    opts = [
+      bindings: state.bindings,
+      file: "repl"
+    ]
+
+    case CljElixir.Compiler.eval_string(source, opts) do
+      {:ok, result, new_bindings} ->
+        result_str = CljElixir.Printer.pr_str(result)
+
+        new_state = %{state |
+          bindings: new_bindings,
+          history: [source | state.history],
+          counter: state.counter + 1
+        }
+
+        {:ok, result_str, new_state}
+
+      {:error, errors} ->
+        error_str = format_errors(errors)
+        new_state = %{state | counter: state.counter + 1}
+        {:error, error_str, new_state}
+    end
+  end
+
+  @doc "Check if input has balanced delimiters (parens, brackets, braces)"
+  def balanced?(input) do
+    input
+    |> String.graphemes()
+    |> count_delimiters(0, 0, 0, false, false)
+  end
+
+  # Count open/close delimiters, respecting strings and comments
+  defp count_delimiters([], parens, brackets, braces, _in_string, _escape) do
+    parens == 0 and brackets == 0 and braces == 0
+  end
+
+  defp count_delimiters([char | rest], p, b, br, in_string, escape) do
+    cond do
+      escape ->
+        # Previous char was \, skip this one
+        count_delimiters(rest, p, b, br, in_string, false)
+
+      char == "\\" and in_string ->
+        count_delimiters(rest, p, b, br, in_string, true)
+
+      char == "\"" and not in_string ->
+        count_delimiters(rest, p, b, br, true, false)
+
+      char == "\"" and in_string ->
+        count_delimiters(rest, p, b, br, false, false)
+
+      in_string ->
+        count_delimiters(rest, p, b, br, true, false)
+
+      char == ";" ->
+        # Comment - skip rest of line
+        rest_after_newline = Enum.drop_while(rest, &(&1 != "\n"))
+        count_delimiters(rest_after_newline, p, b, br, false, false)
+
+      char == "(" -> count_delimiters(rest, p + 1, b, br, false, false)
+      char == ")" -> count_delimiters(rest, p - 1, b, br, false, false)
+      char == "[" -> count_delimiters(rest, p, b + 1, br, false, false)
+      char == "]" -> count_delimiters(rest, p, b - 1, br, false, false)
+      char == "{" -> count_delimiters(rest, p, b, br + 1, false, false)
+      char == "}" -> count_delimiters(rest, p, b, br - 1, false, false)
+
+      true -> count_delimiters(rest, p, b, br, false, false)
+    end
+  end
+
+  defp format_errors(errors) when is_list(errors) do
+    Enum.map_join(errors, "\n", fn
+      %{message: msg, line: line} when is_integer(line) and line > 0 ->
+        "Error on line #{line}: #{msg}"
+
+      %{message: msg} ->
+        "Error: #{msg}"
+
+      other ->
+        "Error: #{inspect(other)}"
+    end)
+  end
+
+  defp format_errors(other) do
+    "Error: #{inspect(other)}"
+  end
+end