feat(code-runtime-python): add the fd-3 frame protocol

Introduce @deepseek-ai/dsh-code-runtime-python with the versionless
JSON-lines protocol between the Node host and the CPython subprocess:
the host-side hostile-frame codec (validateChildFrame, encodeJsonPlain,
checkDoneValue, hasUnsafeIntegerToken, hasNonLosslessNumber,
logTruncationMarker) and the Python-side wire-vocabulary mirror
(py/protocol.py).

This is the protocol layer of the code-runtime-python stack, split from
#436 and based on the multi-language seam extension. The PythonCodeRuntime
implementation and its Python JSON codec land in the backend-core PR on
top of this branch.

Ship the minimal buildable package skeleton (package.json, tsconfig,
tsdown, barrel index, invariant companion, bilingual README) because the
workspace-constraint, coverage, and invariant-topology gates require the
package to exist and build the moment its directory does; the backend-core
PR extends those files rather than creating them.

Align py/protocol.py with src/protocol.ts (the round-12 review of #436
found LogMessage.truncated, DoneMessage.error.kind, and Namespace.errorClass
stale) and guard the two runtime-executed surfaces (PROTOCOL_FD and the log
truncation marker) with a real-python3 cross-language mirror e2e test.

packages/code-runtime/code-runtime-python/src/index.ts

@@ -0,0 +1,20 @@
+/**
+ * CPython subprocess code runtime for the DeepSeek Harness code-execution seam.
+ *
+ * This layer of the package ships the versionless fd-3 wire protocol between the
+ * Node host and the CPython subprocess; the `PythonCodeRuntime` implementation
+ * that drives a `python3 -I` process over it lands on top of this seam. The
+ * protocol's host-side codec and hostile-frame validators are re-exported so the
+ * runtime and its tests share one wire vocabulary.
+ * @module @deepseek-ai/dsh-code-runtime-python
+ */
+
+export type { BootMessage, ChildToHost, ReplyMessage } from './protocol.ts'
+export {
+  checkDoneValue,
+  encodeJsonPlain,
+  hasNonLosslessNumber,
+  hasUnsafeIntegerToken,
+  logTruncationMarker,
+  validateChildFrame,
+} from './protocol.ts'