fix(code-runtime-python): reserve the log array envelope byte, unblock SIGXCPU before re-raise

Addresses the review's two remaining code warnings and the three suggestions:
- Log ledgers (host and child) start one byte below the budget, reserving the
  serialized outer-array envelope (two brackets and n-1 commas over n entries'
  separators); the exact-zero test moves to maxLogBytes 104 and a new exact-limit
  case pins that maxLogBytes 5 admits ['a'] (5 bytes) while 4 truncates to the
  marker alone.
- die_if_cpu_exhausted unblocks SIGXCPU (pthread_sigmask SIG_UNBLOCK, captured at
  import, None-guarded for Windows) before re-delivering it, so a program that
  masks SIGXCPU, burns past the soft limit, and returns is still classified as a
  timeout; a regression test pins the masked path.
- ast.parse passes filename="<model>" so parse-time syntax diagnostics carry the
  same source label as compile and runtime tracebacks; the syntax-error test
  asserts the label.
- The NUL-escape test comments use the true six-byte JSON escape \u0000 instead
  of the caret notation; the README Known Limitations (en + zh) records that
  PID-reuse protection is inert on macOS; a combined-rebind regression test pins
  BaseException plus the traceback reporter rebinding together.

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

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # 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: 3522c7726460b10f0366c56566336465a76491c5
-README.zh.md: 95984cf1ae3677aff4a5c4565bed87a57701d086
+README.md: 9184573748a2dd32c97fe92b6ae91ab89279516a
+README.zh.md: ff4698f06004a2da8e77cb8772a2b23d462d2361