Queue-based Multithreaded Frame Export

[cappelletto]

Description
Replace synchronous frame export with a producer–consumer queue. Decoding/processing enqueues; one or more writer threads flush to disk.\n\nExpected Output / Criteria\n- Export queue module/class implemented with graceful shutdown and error propagation.\n- Deterministic output ordering preserved (unit test).\n- ≥30% export throughput improvement vs baseline on a reference dataset.