Move the filter rule-tree/context helpers and entry inspection into
scanner_filter.c, the parallel scanner into scanner_parallel.c, and keep
the sequential scanner in scanner.c. Shared internal declarations live in
the new scanner_internal.h; scanner.h stays the public façade.
Decompose directory_scanner_next into static helpers (skipped-entry,
selection-protection, mount/-x, directory-finish and per-entry handlers)
with no semantic change.
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