docs(cli): describe buffered XML JSON transcoding accurately

[nth-bailey]

Summary

• Describe polyxml transcode as whole-document XML/JSON conversion in CLI help, README, and guides. Stdin/stdout pipes remain supported, but input and output are buffered.
• Correct Rust core API comments and the Rust guide's outdated transcode example, including its function signature and JSON-to-XML root inference.
• Record the memory behavior in the core-engine agent runbook.

Verification

• cargo fmt --check
• cargo test -p polyxml-cli -q (34 integration tests passed)
• cargo run -q -p polyxml-cli -- transcode --help
• uvx zensical build --clean --strict
• Pre-commit and pre-push hooks passed