Replace num_requests threshold with recent TTFT median as migration
trigger. Track per-instance rolling TTFT (last 8 requests) and trigger
migration when median > 5s (configurable). Target is the instance with
lowest recent TTFT, requiring > 2x improvement to justify migration.
This is more responsive than the instantaneous num_requests signal
because TTFT directly measures the user-facing impact of contention.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>