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/tests/protocol-mirror.e2e.ts

@@ -45,7 +45,7 @@ describe.skipIf(!python3Available)('protocol.py mirrors protocol.ts at runtime',
       '  "markers": [log_truncation_marker(b) for b in budgets],',
       '}))',
     ].join('\n')
-    const { stdout } = await execFileAsync('python3', ['-I', '-c', probe])
+    const { stdout } = await execFileAsync('python3', ['-I', '-B', '-c', probe])
     const seen = JSON.parse(stdout) as { fd: number; markers: string[] }
     // Assert against the TS-side PROTOCOL_FD export (the value the host wires),
     // not a bare literal, so a drift on either side of the wire is caught here.
@@ -75,7 +75,7 @@ describe.skipIf(!python3Available)('protocol.py mirrors protocol.ts at runtime',
       + ' if not n.startswith("_") and hasattr(v, "__required_keys__")}',
       'print(json.dumps(frames))',
     ].join('\n')
-    const { stdout } = await execFileAsync('python3', ['-I', '-c', probe])
+    const { stdout } = await execFileAsync('python3', ['-I', '-B', '-c', probe])
     const seen = JSON.parse(stdout) as Record<string, { required: string[]; optional: string[] }>
     // Normalize the TS source of truth to the same sorted shape Python reports.
     const expected = Object.fromEntries(