fix(gateway): skip oversized JSONL lines to prevent event-loop starvation
Large transcript JSONL records (multi-MB tool results, file content)
block the event loop via JSON.parse before truncation logic can skip
them. Add a 256 KiB line-size guard to parseTailTranscriptRecord and
extractUsageSnapshotFromTranscriptLine, and replace the full transcript
index scan in readSessionTitleFieldsFromTranscriptAsync with the
existing bounded sync reader.
Observed improvement on a production install (33 sessions):
chat.history dropped from 13-16s to ~1.2s, event loop utilization
from 0.999 to normal, steady-state CPU from ~100% to 0.2-0.3%.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
