StringDecoder correctly assembles multi-byte chars (Cyrillic) across
stdin chunk boundaries. Closes Windows Node quirk where Russian prompts
were turned into mojibake before sending to Anthropic API (Layer 2 escalation).
Stage 3 follow-up fix 1/3 (helper).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Дмитрий