fix(node:stream): decode setEncoding buffer chunks

[andrewtdiz]

Summary

• decode readable Buffer chunks through the active readable encoding before buffering or data-event emission
• preserve Node-like base64 StringDecoder chunking by carrying remainder bytes until EOF flush
• return strings from no-size read() when readableEncoding is active and remove the now-passing known-failure entries

Verification

• cargo test -p perry-runtime readable_set_encoding
• cargo fmt --check
• cargo check -p perry-runtime -p perry-stdlib -p perry-codegen
• jq empty test-parity/known_failures.json
• git diff --check
• ./run_parity_tests.sh --suite=node-suite --module=stream --filter encoding/base64
• ./run_parity_tests.sh --suite=node-suite --module=stream --filter encoding/hex
• ./run_parity_tests.sh --suite=node-suite --module=stream --filter encoding/latin1
• ./run_parity_tests.sh --suite=node-suite --module=stream --filter readable/set-encoding

Residual: readable/push-string-with-encoding-set still fails and remains in known_failures.json.