docs: fix determinism return-value-passing example (closes #217)

[yaythomas]

Summary

Fixes the "Pass data through return values, not closures" example so it demonstrates the failure mode the surrounding prose describes.

The current wrong example accumulates total from item["price"] in event["items"]. Both values are pure functions of the handler input, which is stable across replays. Replay reconstructs the same total. The example matches the page's own rule for allowed outside-step code rather than the anti-pattern the prose is teaching.

The new wrong example writes to an outer list from inside the step body via a closure. Replay returns the cached value without running the body, so the write does not land and the list stays empty. The handler's return value differs between the first invocation and replay. The new right example returns the receipt id from the step and appends it to the list in the handler.

Changes

• Rewrite the section prose and the danger admonition to describe the real failure mode. Steps checkpoint their return value. Replay returns the cached value in place of running the body. Any state a step body writes to outer variables through a closure disappears on replay.
• Replace the TypeScript and Python return-value-passing wrong and right examples.
• Add a wrong companion to the Java return-value-passing example, which previously showed only the right implementation.

Trace on replay (Python wrong version)

receipts: list[str] = []                       # reinitializes
for item in event["items"]:
    context.step(save_and_track(item, receipts), name=...)
    # step returns cached None on replay
    # save_and_track body does not run
    # receipts.append never fires
return {"receipts": receipts}                  # []

First invocation returns {"receipts": [id1, id2, ...]}. Replay returns {"receipts": []}. Same shape for TypeScript and Java.

Verification

• Ran mdformat on the edited page.
• Ran zensical build --clean. Build finished cleanly, no issues.
• Verified context.step signatures against SDK source for all three languages before writing examples.

Closes

• Closes docs: "Pass data through return values, not closures" example does not demonstrate the failure it warns about #217