- oakengine_export_render drives ExportTask synchronously (offline
render + encode) with a progress callback, codec probing, and a
thread-local error channel; exporter.h keeps clear of the visibility
macro header
- oak-cli transcode now defaults to mp4 (H.264/AAC) with --format ppm
keeping the raw output path
- two real concurrency bugs found by the facade's own test: ExportTask
deadlocks when start()ed synchronously (queued conform handshake
needs an event loop), and the progress callback must be captured by
value because it fires on the task thread
