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.zh.md

@@ -39,4 +39,3 @@ host 与 CPython 子进程在子进程的 fd 3 上交换一个无版本号的 JS
 - **调用 `setsid()` ／ `start_new_session=True` 的后代会逃出 teardown。** 终止是用 `kill(-pid)` 向子进程的进程组发信号；一个把自己移入新会话的后代已不在该进程组内，任何信号都到不了它。若它同时释放了继承而来的 stdout／stderr／fd-3 管道，leader 的 `close` 仍会结算该次运行，在 `closeDeadline` 到界之后 fiber 变为完全停稳，而那个孤儿仍在运行。这是 containment 边界，而非安全边界——模型代码具有等同 bash 的信任级别，一个 bash 工具同样能 `setsid` 逃逸。要够到这样的孤儿需要追踪每一个后代 pid（如 bash-local 后端的 process-inspector 所做），此项已推迟；进程组 teardown 会回收所有留在组内的进程。
 - **日志与完成值的叠加峰值未被加载门建模。** 每项预算都是各自对照 `addressSpaceMb` 检查的。模型的 daemon 线程可以在完成值被计量并分帧的窗口内持续写入、把日志 pending 重填到接近 `maxLogBytes`，于是两个峰值以任何门都不曾放行也不曾拒绝的方式相加。对 `(maxLogBytes + maxValueBytes)` 设门的方案经评估后推迟：它的判别用例无法在 `RLIMIT_AS` 之下确定性地构造出来，因此该门只能证明自己的算术。叠加峰值被触及时该次运行死为 `worker-exit`——containment 仍然成立，只是失败分类失真。
 - **宽 binding 回复会按成员展开宿主侧状态。** 回复经由 [`@deepseek-ai/dsh-session`](../../core/session/README.md) 的 `snapshotJsonValue` 穿越，其 `walkJsonValue` 为每个成员压入一个任务帧，而 binding 回复在 seam 层没有字节上限。因此一个数百万元素的合法回复可以耗尽宿主堆。该性质属于那个共享遍历，而不属于本后端——worker-thread 后端消费同一个函数——所以修复应落在 `packages/core/session`，让所有消费方一并受益。
-- **并发 binding 回复没有对 fd 3 做节流。** 管道缓冲写满后 `proto.write` 返回 `false`，而本后端不等待 `drain`，因此在一轮 `asyncio.gather` 中多个 binding 同时返回大值时，它们会一起编码并排入宿主内存。把回复串行化可以给它设界，代价是改变 seam 当前允许的并发度；同类的 worker-thread 后端没有等价物（它投递结构化克隆，不存在流背压），因此仓库内没有可照抄的先例。