fix(code-runtime-python): clear stray buffers on truncation and drain the original std streams

Addresses the review's two carried warnings and the comment suggestion:
- Once the ledger truncates, every arm that marks it (admit()'s two ceilings and
  the child-marker frame arm) now clears both stray pipes' buffered output
  wholesale, so the end-path flushStray sees empty buffers instead of
  concat+decoding doomed data near a 256 MiB maxLogBytes; captureStray's newline
  loop re-checks the flag before re-retaining the residual.
- The child runs with -u (unbuffered), so sys.__stdout__/sys.__stderr__ writes
  are visible to stray capture immediately; the settlement flush still drains
  the original std streams before the done frame as a guard. A regression test
  writes through sys.__stdout__/sys.__stderr__ without an explicit flush and
  asserts both bytes land in logs. C-ext stdio remains an accepted residual,
  recorded in the README Known Limitations (en + zh).
- The ledger-comment arithmetic now states the exact boundary (serializes to
  exactly maxLogBytes; without the reserved byte it would be maxLogBytes + 1)
  in both host and child.
Note (en + zh) registers the stray-clear and -u/settlement-drain mechanisms and
the new test; pairings re-recorded; corpus passes 1029.

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: fbb37d36523d12357a30c319409afc69ce64e487
-README.zh.md: 10141bc634e523b1fa079280039b0e5eda7e45d8
+README.md: 25d56e1e611a04523d933a16784e51a91d689fa3
+README.zh.md: 978a875c815d33dcfadfd5114ee8b88dbde4fc92