refactor(scanner): split into filter/sequential/parallel TUs

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.

CMakeLists.txt

@@ -139,6 +139,8 @@ set(CLIENT_CORE_SRCS
 	src/client/client_send.c
 	src/client/client_validation.c
 	src/client/scanner.c
+	src/client/scanner_filter.c
+	src/client/scanner_parallel.c
 	src/client/usage.c
 )
 set(CLIENT_MAIN_SRCS src/client/client_cli.c)