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/tests/protocol.spec.ts

@@ -135,8 +135,8 @@ describe('lossless-number scan', () => {
   it('walks wide arrays and objects one member at a time', () => {
     // `call.args` carries no seam byte cap, so a wide forged payload has no
     // budget to be rejected against — the walk must hold one cursor per
-    // NESTING LEVEL, not one entry per member, or a flat payload just below
-    // the 256 MiB frame ceiling would allocate tens of millions of stack
+    // NESTING LEVEL, not one entry per member, or a flat payload at the top of
+    // the host's inbound frame-size cap would allocate tens of millions of stack
     // entries (and `Object.values` a second full-breadth copy). Observable
     // through the boundary: a wide payload whose per-member cost the old shape
     // would have paid still scans, and a violation ANYWHERE in it is found