fix(code-runtime-python): make the write-path pre-checks open-aware; document the open flag in zh

The review's warning: while an open entry accumulates, the newline pre-checks in
the write path still charged a NEW entry's +3 cheap-bound overhead (quotes +
separator), so an exact-fit merged TAIL was truncated (or the pre-check
over-rejected it and flushed a truncated prefix). Both pre-checks now charge
the overhead only when no open entry is in progress, matching _push_locked's
open-aware bound. A regression case (the review's recipe: flush an open
fragment, then write one exact-fit newline-terminated line) is verified to
truncate when the +3 is restored.

The zh README's wire-contract section now describes the open flag like the en
side (the fd-3 Agent Note holds the split-billing arithmetic; a cross-doc link
was omitted to keep the bilingual link sequence aligned).

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

@@ -296,7 +296,14 @@ class _LogStream(io.TextIOBase):
             pos = 0
             if self._pending or self._pending_blocks:
                 newline = text.index("\n")
-                if self._pending_chars + newline + 3 > self._logs.remaining:
+                # The +3 cheap-bound overhead (quotes + separator) belongs to a
+                # NEW entry. While an `open` entry is accumulating, the closing
+                # line is that entry's TAIL: its cheap bound is the content
+                # length alone, matching `_push_locked`'s open-aware bound.
+                # Charging +3 here truncates an exact-fit merged tail or
+                # over-rejects it, then flushes a truncated prefix instead.
+                overhead = 3 if not self._logs._open_started else 0
+                if self._pending_chars + newline + overhead > self._logs.remaining:
                     # The reconstructed first line cannot fit the ledger, so
                     # LogBuffer would reject it whole: copy only the prefix that
                     # fails its cheap bound and drop the chunks. The slice is
@@ -331,7 +338,8 @@ class _LogStream(io.TextIOBase):
                 # prefix, which push still rejects on its own cheap bound (the
                 # prefix is longer than `remaining`), so the marker is emitted
                 # and the oversized line is never materialized.
-                if newline - pos + 3 > self._logs.remaining:
+                overhead = 3 if not self._logs._open_started else 0
+                if newline - pos + overhead > self._logs.remaining:
                     self._logs.push(text[pos:pos + self._logs.remaining + 4])
                     break
                 self._logs.push(text[pos:newline])