Supervised, bounded asynchronous observability export bus.

Admission is bounded by event count and bytes. Export workers are bounded by count, time and heap, and their mailbox payloads are kept off heap. The delivery guarantee is bounded best effort: uncertain failures are retried after bounded exponential backoff and can duplicate an event, while queue pressure or attempt exhaustion can still result in zero successful exports. Exporters should use trace/span/event identity when they need deduplication.