Buffer and sort each directory's inspected entries (non-directories
ascending, then directories ascending) and walk them depth-first via a
LIFO directory stack, so the sequential scanner's stream matches rsync
3.4.1's flist order. This makes the --info=name transfer order and the
--delete-during/--delete-delay deletion sequence byte-identical to rsync
(differential tests in test_parity_order.py); --threads stays unordered
(no rsync analogue) and is documented as such.
Adds LIFO queue_push/queue_pop over the existing ring buffer.
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