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/invariant.ts

@@ -0,0 +1,30 @@
+/**
+ * Package-owned invariant companion for `@deepseek-ai/dsh-code-runtime-python`.
+ * @module @deepseek-ai/dsh-code-runtime-python/invariant
+ */
+
+/* jscpd:ignore-start */
+import type { Context } from 'cordis'
+import type { InvariantInstaller } from '@deepseek-ai/dsh-invariants'
+
+const PACKAGE_NAME = '@deepseek-ai/dsh-code-runtime-python'
+
+/** Cordis companion plugin name. */
+export const name = 'code-runtime-python-invariant'
+/** Service required before the companion can reserve package ownership. */
+export const inject = ['invariants']
+
+/**
+ * No runtime invariant: this process-boundary implementation exposes no same-process event relation;
+ * the fd-3 protocol and real-subprocess integration tests cover it.
+ */
+const install: InvariantInstaller = () => {}
+
+/**
+ * Register this package's invariant companion.
+ * @param ctx - Cordis context carrying the invariant service.
+ * @returns the installed registration's disposer after setup succeeds.
+ */
+export const apply = (ctx: Context): Promise<() => void> =>
+  Promise.resolve(ctx.invariants.register(PACKAGE_NAME, install))
+/* jscpd:ignore-end */