Unknown files are no longer terminal. Stage 1 now routes :unknown onto a
dedicated queue (with the same blocking backpressure as the known queue),
and a third worker pool sorts each file into data/binary/ or data/text/
using a NUL-byte sniff of the first 8000 bytes.
- content.jl: is_binary content sniff (stage 3)
- worker.jl: handle_unknown_job; stage-1 routes unknown with backpressure;
KNOWN_ENQUEUE_RETRY_SECONDS -> ROUTE_ENQUEUE_RETRY_SECONDS (serves both)
- config.jl: unknown_worker_count/queue_capacity, binary_dir, text_dir + env
- FileServer.jl: unknown queue, pool, stage-aware recovery, drain ordering
- tests for is_binary and handle_unknown_job; tmp_config isolates new dirs
- README: three-stage pipeline