Minimal CPython REPL runtime (rlm.repl)

[snimu]

Adds rlm.repl: a minimal CPython REPL runtime (one 635-line file + a 127-line protocol doc) that speaks newline-delimited JSON over stdio — execute / interrupt / snapshot / restore / shutdown in, streamed stdout / stderr / result / display / error / done events out.

Design points:

• Top-level await (PyCF_ALLOW_TOP_LEVEL_AWAIT) on one persistent event loop; background tasks keep running between cells; last-expression repr echo like a REPL.
• Clean CPython tracebacks: cell sources registered in linecache (so tracebacks show the offending line), runtime-internal frames stripped, no ANSI decoration.
• The protocol channel lives on a private dup of fd 1 while user fds are piped through pump threads — even C extensions writing to fd 1 cannot corrupt framing.
• Interrupts work for both sync-blocking and await-suspended cells (SIGINT dispatch on the main thread with task-cancel fallback); interrupts for not-yet-started requests are parked and delivered at activation.
• First-class snapshot/restore with dill, matching the existing snapshot semantics; restoring old IPython-era snapshots skips In/Out/get_ipython.
• bash() from the previous PR works unchanged on the persistent loop.

Standalone in this PR: the runtime is fully tested (25 new tests incl. both interrupt paths, direct-fd capture, framing under noise) but the host still runs ipykernel — the host swap is the next PR. Measured: ~30 ms start-to-ready (ipykernel: ~1.2 s), ~0.13 ms per-cell round-trip.

Ticket: Linear ticket to be linked shortly (Linear API auth is being restored).

Stack

This is PR 2 of 4; each PR merges into its predecessor (merge bottom-up):

1. Async bash() tool for the kernel runtime #1684 — Async bash() tool for the kernel runtime
2. Minimal CPython REPL runtime (rlm.repl) #1685 — Minimal CPython REPL runtime (rlm.repl)
3. Run the kernel on the REPL runtime #1686 — Run the kernel on the REPL runtime
4. Complete the REPL cutover: remove IPython, ZMQ, and cell magics #1687 — Complete the REPL cutover: remove IPython, ZMQ, and cell magics

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Note

Medium Risk
New arbitrary-code execution runtime with SIGINT dispatch, fd remapping, and dill pickle restore. Not yet the production kernel path, but bugs here will become session-critical once the host swaps over.

Overview
Adds a standalone python -m rlm.repl runtime that speaks newline-delimited JSON over stdio, as a future replacement for ipykernel (host still uses Jupyter in this PR).

Cells run in one persistent __main__ namespace on a single asyncio loop with top-level await, trailing-expression repr, captured stdout/stderr (including direct fd writes), emit() display events, and KeyboardInterrupt for both sync-blocked and await-suspended work. Snapshot/restore pickles the user namespace with dill (lazy import; size caps and optional prune). Shutdown kills live bash() children.

Also delays ipykernel/IPython imports so the package can load outside a Jupyter kernel, documents the protocol in repl.md, and covers the runtime with subprocess tests.

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Note

Add rlm.repl CPython REPL runtime with JSON-lines protocol

• New rlm.repl module implements a subprocess-based REPL runtime speaking a newline-delimited JSON protocol over stdin/stdout, supporting cell execution with trailing-expression result echo, top-level await, and output capture via pipe pumps.
• Interrupt handling covers sync-blocked, await-suspended, and background-task-hogged scenarios via a SIGINT handler that raises KeyboardInterrupt into the active task or cancels and re-targets as needed.
• Snapshot/restore support uses dill to serialize namespace variables with per-variable and aggregate byte caps, manifest generation, and prune-oversized reporting.
• Refactors rlm.__init__.py to defer ipykernel.comm.Comm and IPython.get_ipython imports to call time, so importing rlm no longer fails in environments without ipykernel.
• Adds comprehensive protocol docs in repl.md and a full test suite in test_repl.py.
• Risk: rlm.host_request now raises RuntimeError at call time (not import time) when ipykernel.comm.Comm is unavailable; callers that previously relied on import-time failure will see the error later.

^{Macroscope summarized e017cad.}

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