Rewrite the posix Walk implementation to avoid the extra ReadDir per
directory that was noted as a TODO in the old code. The new algorithm
holds all traversal state in a walkState struct and uses processDir to
interleave sibling entries in correct S3 lexicographic order without a
second syscall.
Key changes:
prefix optimisation: jump directly into the deepest matching directory
rather than scanning from the root on every call
marker short-circuit: skip entire subtrees that are lexically before
the marker, making paginated listing faster
Co-authored-by: Ben McClelland <ben.mcclelland@versity.com>
tonyipm