feat(code-runtime-python): make the TypedDict wire mirror an executable gate

Address the two standing review suggestions in this layer rather than deferring
them to PR #4:

- Extend tests/protocol-mirror.e2e.ts to read each py/protocol.py TypedDict's
  required/optional key set and assert it against the wire field names
  src/protocol.ts declares (global included, via functional TypedDict). The
  round-12 class of drift — a renamed/dropped field, or one side making a field
  optional the other requires — now fails a test instead of relying on review.
  Field types remain review-guarded (no mechanical TS/Python equivalent).
- Drop the forward references to PR #4's internal mechanisms from this layer's
  prose: the "256 MiB frame ceiling" figure and the "(index.ts)" fd-3 pinning
  citation become an abstract "host-side inbound frame-size cap" so the JSDoc,
  spec, README, and Agent Note describe only what this layer owns.

Update both README sides and the Agent Note (both languages) to state the
mirror is now executable, and re-record their i18n pairings.

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: 62e899941ac8eadb2046b1bffae0a3c9c6e14308
-README.zh.md: 5a7aa880830bccc4ebd647d7b009ec7e4f0d6307
+README.md: d0491c478d04a8436199bc23fd79917e8c019b1c
+README.zh.md: 63d7d38ee05b561e60a3adf387c5c1292c37a7a2