carry trailing high surrogate across CodePointBuffer appends

[rootvector2]

appendArrayInt emitted a chunk-trailing high surrogate immediately instead of holding it in prevHighSurrogate, so a surrogate pair split across two append() calls (e.g. CharStreams.fromReader filling its read buffer mid-emoji) decoded to a spurious lone surrogate before the real code point; found tracing the chunked decode path, the fix keeps it pending for the next append() and flushes a still-dangling one in build().