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/tests/runtime.spec.ts

@@ -1995,6 +1995,28 @@ describe('PythonCodeRuntime — programs and bindings', () => {
     expect(result.logs).toEqual(['a'.repeat(30) + 'b'.repeat(30)])
   }, 15_000)
 
+  it('does not over-reject an exact-fit closing line while an open entry accumulates', async () => {
+    // The write-path pre-check's cheap bound used +3 (quotes + separator) even
+    // while an open entry was accumulating, so an exact-fit merged TAIL was
+    // truncated: print('a'*29, flush) bills 32 (ledger 31 left), then
+    // print('b'*28, end=''); print('c') merges 29 more chars whose cheap bound
+    // is 29, not 32 — the +3 form saw 28 + 1 + 3 = 32 > 31 and truncated a
+    // line that fits (merged cost 2 + 58 + 1 = 61 <= 63).
+    const { runtime } = await setup({ maxLogBytes: 64 })
+    const result = await runtime.run({
+      program: [
+        'import sys',
+        "sys.stdout.write('a' * 29)",
+        'sys.stdout.flush()',
+        "sys.stdout.write('b' * 30 + chr(10))",
+        'return "done"',
+      ].join('\n'),
+      bindings: [],
+    })
+    expect(result.error).toBeUndefined()
+    expect(result.logs).toEqual(['a'.repeat(29) + 'b'.repeat(30)])
+  }, 15_000)
+
   it('rejects a new open entry once the ledger has only two bytes left', async () => {
     // The jsonStringCostUpTo sub-2-byte guard: forged open frames drive the
     // host ledger down to 1 byte, then a new open entry's first-fragment cap