Replace two-phase (prepare all, then encode+write) with sliding-window
pipeline. Maintains bounded in-flight window (max_workers * 4 = 16),
emits EMBED_PROGRESS from the first paper completion (~2s vs ~30s).
Key changes:
- producer/consumer loop: prepare + submit one paper at a time
- wait(FIRST_COMPLETED) in main thread for encode results
- processed_count = skip + embedded, monotonic EMBED_PROGRESS
- resume skip and no-payload paths also advance processed_count
- encode failure fails closed (return 1, no silent skip)
- removed PR9B_BATCH_SIZE and _batched() (replaced by sliding window)
- 4 integration tests for progress counting, failure, and write cycle
db2988d8f0