fix(code-runtime-python): verify union<->roster both ways, stop pycache writes, refresh metering prose

Address the latest review round:

- WireFrameShapesCoverUnions checked only union ⊆ roster, so removing a frame
  from a message union (e.g. dropping ReplyErr from ReplyMessage) left the check
  true while the public TS union diverged from the wire. Replace it with a
  bidirectional equivalence between MessageFrames and the roster's message-frame
  value types (nested Namespace/ErrorClass/DoneErrorField excluded): both a frame
  added to a union without a roster entry and a frame removed from a union now
  fail typecheck (both verified).
- The mirror e2e's python3 probes imported protocol.py without -B, writing
  py/__pycache__/*.pyc into the (un-ignored) source tree. Add -B to both.
- Refresh the metering prose (checkDoneValue JSDoc + README both sides + Agent
  Note both sides): the incremental-work list no longer says "per-key
  JSON.stringify" now that jsonStringBytesUpTo scans without stringifying;
  re-record the README and Agent Note 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: d0491c478d04a8436199bc23fd79917e8c019b1c
-README.zh.md: 63d7d38ee05b561e60a3adf387c5c1292c37a7a2
+README.md: 606d153eb899925ae274133b3728d317607a17e2
+README.zh.md: f4387d6b20994781b8e64d31da7319badb56aacd