Implements a full-featured REPL using an accumulate-and-recompile
strategy with output diffing. Auto-imports std, auto-prints bare
expressions, supports multi-line input, tab completion, and REPL
commands (:help, :quit, :clear, :source, :history, :type).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>