fix(code-runtime-python): reset the inherited SIGXCPU disposition and mask at startup

The reviewer's standing issue: the child inherits the host's SIGXCPU
disposition and signal mask — if the host ignores or blocks SIGXCPU, the soft
RLIMIT_CPU fires but cannot stop the child, and the hard limit's SIGKILL then
classifies a definite CPU overrun as worker-exit instead of a timeout. The
bootstrap now resets SIGXCPU to SIG_DFL and unblocks it before any model code
runs (the settle-time enforcer already restores SIG_DFL for a program that
traps or masks the signal mid-run; this closes the inherited-state gap). The
zh README's outer wire section also gains the truncation-exception sentence to
match the en side.

packages/code-runtime/code-runtime-python/README.i18n.yaml

@@ -3,4 +3,4 @@
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/code-runtime/code-runtime-python/README.md
 README.md: 2e22a5122f891b8317e13ee395ce222805b01b29
-README.zh.md: 0f8773bc8e8292cfc3f0b09395d46ff75b17392c
+README.zh.md: 028d2f5f7b57f1ca4da46557ba66511992227379

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

@@ -55,7 +55,7 @@ kind: "package-reference"
 
 ### wire 契约
 
-帧为 `boot`／`run`（宿主 → 子进程）与 `boot-ack`／`call`／`log`／`done` 加每个 call 一个 `reply`（子进程 → 宿主）。`log` 帧的 `truncated` 标志标记的就是子进程账本自己的截断标记帧，因此宿主在与子进程相同的点停止捕获，而不是从自己的预算推断。`log` 帧的 `open` 标志标记由显式 flush 提交的未结束行：宿主把下一个 log 帧合并进同一条目，因此 `print('a', end='', flush=True); print('b')` 读回为一条 `'ab'` 条目而不是假换行（拆分计费算术在 fd-3 协议 Agent Note 的 wire-contract 段）。合并的唯一例外是截断：当后续超预算帧触发账本时，已计费的前缀作为独立条目先提交，截断 marker 跟在后面（marker 保持末位，无重复计费）。`done.error.kind` 为 `exception`、`invalid-output`、`output-limit` 之一；墙钟／CPU 预算、中止与基底死亡在宿主侧观察，不以帧形式携带。
+帧为 `boot`／`run`（宿主 → 子进程）与 `boot-ack`／`call`／`log`／`done` 加每个 call 一个 `reply`（子进程 → 宿主）。`log` 帧的 `truncated` 标志标记的就是子进程账本自己的截断标记帧，因此宿主在与子进程相同的点停止捕获，而不是从自己的预算推断。`log` 帧的 `open` 标志标记由显式 flush 提交的未结束行：宿主把下一个 log 帧合并进同一条目，因此 `print('a', end='', flush=True); print('b')` 读回为一条 `'ab'` 条目而不是假换行（拆分计费算术在 fd-3 协议 Agent Note 的 wire-contract 段）。合并的唯一例外是截断：当后续超预算帧触发账本时，已计费的前缀作为独立条目先提交，截断 marker 跟在后面（marker 保持末位，无重复计费）。合并的唯一例外是截断：当后续超预算帧触发账本时，已计费的前缀作为独立条目先提交，截断 marker 跟在后面（marker 保持末位，无重复计费）。`done.error.kind` 为 `exception`、`invalid-output`、`output-limit` 之一；墙钟／CPU 预算、中止与基底死亡在宿主侧观察，不以帧形式携带。
 
 ### 无损 JSON 跨越
 

packages/code-runtime/code-runtime-python/py/bootstrap.py

@@ -1195,6 +1195,17 @@ async def _run(channel: ProtocolChannel) -> None:
             # can `except ToolCallError as e:` and read the member property.
             namespaces[declared["name"]] = error_class
 
+    # The child inherits the host's SIGXCPU disposition and signal mask. If
+    # the host ignores or blocks SIGXCPU, the soft RLIMIT_CPU fires but cannot
+    # stop the child — the hard limit's SIGKILL then classifies a definite CPU
+    # overrun as substrate death (worker-exit) instead of a timeout. Reset to
+    # the default disposition and unblock before any model code runs (the
+    # settle-time enforcer already restores SIG_DFL for a program that traps or
+    # masks the signal mid-run; this closes the inherited-state gap).
+    signal.signal(signal.SIGXCPU, signal.SIG_DFL)
+    if getattr(signal, "pthread_sigmask", None) is not None:
+        signal.pthread_sigmask(signal.SIG_UNBLOCK, (signal.SIGXCPU,))
+
     channel.send_sync({"type": "boot-ack"})
 
     # 3. Start a reply-pump task before the run message: replies can arrive