fix(code-runtime-python): send the run frame after boot-ack; reject directories in pythonBin resolution

The review's two behavior items: the run frame was written back-to-back with
the boot frame (the seam contract puts run after boot-ack, which confirms the
namespaces were accepted); it now goes out from the boot-ack handler, so a
boot failure cannot race the run frame. resolvePythonBin now requires the
candidate to be a regular file — a directory passes X_OK and would otherwise
shadow a later real interpreter. Doc spots: the load-time overflow message
says worker-exit (not stranding to the wall clock), the run JSDoc spells out
the resolve-with-error contract, the PATH-stub test removes the stale v8
ignore, and the README's binding-value bullet names serialization cost.

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

@@ -116,7 +116,7 @@ kind: "package-reference"
 - **原始长度超过 64 MiB 的 fd-3 帧会让本次运行以 worker-exit 结算**——`maxLogBytes`/`maxValueBytes` 在加载期被限制到同一解析器上限，因此诚实子进程的帧总能放得下；模型构造的超过 64 MiB 的 binding 实参（一个在 seam 层没有预算的值）会触发同一上限——这是该 OOM 防护的已接受残余。
 - **组合日志与值的峰值不被加载门建模**——持续写入的模型 daemon 线程与完成值计量、分帧相加的峰值没有任何门会放行或拒绝；运行以 `worker-exit` 告终，隔离成立，只有失败分类降级。
 - **1 秒双限 `ulimit -t 1` CPU 超限被报告为 `worker-exit` 而非 timeout**——当宿主在一个与软限相等的硬 CPU 限下启动且该限为 1 时，`_clamped` 无法下调软限，内核在同一 tick SIGKILL 忙循环，SIGXCPU 永远不会送达；隔离成立，只有分类降级。
-- **中间 binding 值没有字节上限**——实现仍受 structured-clone 成本与进程内存约束，提供方或执行器可能应用自己的获取上限。
+- **中间 binding 值没有字节上限**——实现仍受无损 JSON 序列化成本与进程内存约束，提供方或执行器可能应用自己的获取上限。
 
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 ### 开发备注