Extract the SSE chunk-buffering from sdk.ts into a pure SSEParser (sdk.ts delegates;
behavior unchanged) and pin the framing rules that are easy to break when an event
splits across network reads: partial trailing lines held until completed, frames
reassembled across 2-3 reads, [DONE] sentinel and empty/non-data lines filtered,
each frame emitted exactly once. typecheck clean.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
