orly-repl: don't abort in-flight entries on stdin EOF; let exit flush (#542)

[ohohoreilly]

Closes #542. Unblocks #539 (its docker gate is what exposed this).

Bug

printf '40 + 2;\n:quit\n' | orly-repl ... printed the banner and one prompt, then exited 0 with no result and no error. Two defects compounding:

1. readline's close (stdin EOF) fired while the first entry was still compiling, and the handler quit immediately — closing the WebSocket and deleting the scratch dir under the in-flight entry.
2. process.exit(0) truncated the aborted entry's pending stderr write, making the failure silent.

Interactive use never hits it (stdin stays open), and the existing smoke held its stdin pipe open until after :quit — which is why CI on #536 was green while #539's printf | docker run -i gate failed.

Fix

• Stdin EOF chains quit() onto the serialized entry queue: quit happens after pending entries drain. Ctrl-D semantics unchanged.
• quit() sets process.exitCode instead of calling process.exit; with stdin, readline, and the socket closed, the event loop drains naturally and output always flushes.

Verification

• Smoke gains a piped-EOF regression scenario (write expression, close stdin immediately, require the result + exit 0); full smoke passes locally.
• The exact failing invocation from the docker: repl entrypoint mode — orly-repl baked into the image (#538) #539 gate re-run inside the published docker image: orly> 42, exit 0.