fix(code-runtime-python): pace concurrent binding replies against fd 3

`sendReply` ignored `proto.write`'s `false` return, so a program resolving
several large values in one `asyncio.gather` round encoded every reply in the
same turn and queued all of them in fd 3's writable buffer. Binding resolution
carries no seam-level byte cap to bound that, and the failure kills the host
process rather than failing the run: measured on a 64 KiB-highWaterMark pipe,
eight 4 MiB replies buffered 32.0 MiB at once against 0.0 MiB once paced.

Replies now go through a queue that encodes and writes one frame at a time,
awaiting `drain` when the pipe is full. The encode happens inside the loop, so a
queued reply the run no longer needs is dropped by the `settled` check without
ever being serialized.

This was previously deferred on the grounds that serializing would narrow the
seam's concurrency contract. That reasoning was wrong: the child matches each
reply to its `call` by id from a pump that reads fd 3 continuously, so arrival
order was never observable, and the bindings still run concurrently. Only the
host's peak memory and the flush timing change. The README entry recording the
deferral is removed and the Agent Note records the mechanism instead.

packages/code-runtime/code-runtime-python/README.md

@@ -39,4 +39,3 @@ No direct invalidation; the named consumer owns any request-prefix changes.
 - **A descendant that calls `setsid()` / `start_new_session=True` escapes teardown.** Termination signals the child's process group with `kill(-pid)`; a descendant that moves itself into a fresh session is no longer in that group and no signal reaches it. If it also releases the inherited stdout/stderr/fd-3 pipes, the leader's `close` still settles the run, and after the `closeDeadline` bound the fiber goes quiescent while that orphan keeps running. This is the containment boundary, not a security one — model code has bash-equivalent trust, and a bash tool can `setsid` away just the same. Reaching such an orphan would require tracking every descendant pid (as the bash-local backend's process-inspector does) and is deferred; the process-group teardown reaps everything that stays in the group.
 - **A combined log-and-value peak is not modelled by the load gate.** Each budget is checked against `addressSpaceMb` on its own. A model daemon thread that keeps writing while the completion value is metered and framed can refill the log pending toward `maxLogBytes` during that window, so the two peaks add in a way no gate admits or rejects. A gate over `(maxLogBytes + maxValueBytes)` was considered and deferred: its discriminating case cannot be scheduled deterministically under `RLIMIT_AS`, so the gate would only prove its own arithmetic. When the combined peak is reached the run dies as `worker-exit` -- containment holds and only the failure classification is degraded.
 - **A wide binding REPLY expands host-side state per member.** Resolutions cross through `snapshotJsonValue` in [`@deepseek-ai/dsh-session`](../../core/session/README.md), whose `walkJsonValue` pushes one task frame per member, and binding resolution carries no seam-level byte cap. A legitimate reply of several million elements can therefore exhaust the host heap. The property belongs to that shared walk, not to this backend -- the worker-thread backend consumes the same function -- so the fix belongs in `packages/core/session` where every consumer benefits.
-- **Concurrent binding replies are not paced against fd 3.** `proto.write` returns `false` once the pipe's buffer is full and this backend does not wait for `drain`, so several bindings resolving large values in one `asyncio.gather` round encode and queue together in host memory. Serializing the replies would bound it, at the cost of changing the concurrency the seam currently allows; the sibling worker-thread backend has no equivalent (it posts structured clones, which carry no stream backpressure), so there is no in-repo precedent to copy.