file: add a batch-copy command to the file worker

Batch copy of many small files

On master, CopyJob copies many files one KIO::file_copy job at a time. Each file is its own job and scheduler dispatch plus a serialized command round-trip to the worker, which dominates the wall time for small files where the data copy itself is tiny. This MR adds a file-worker command that copies a list of files in one round-trip, used by CopyJob.

BUG: 342056

What changes

• File worker: a new special() command copies a list of local (source, destination) pairs in one round-trip. It copies with copy_file_range and a read/write fallback, and preserves the same metadata as copy() (permissions, timestamps, ownership, POSIX ACL, extended attributes). It only creates fresh files. An existing destination (or the source itself via a hardlink, or a symlink) is reported back as a conflict and left to copy(). The engine is in file_unix.cpp. file_win.cpp returns ERR_UNSUPPORTED_ACTION.
• CopyJob: when the next files to copy are a run of plain local-to-local files on a local filesystem, it dispatches them as one batch instead of one file_copy job each. Conflicts and per-item errors are requeued and handled by the existing per-file path (conflict dialog, skip, rename), so progress and undo are unchanged.

Performance

Release build. copybatchbench, 5000 x 4 KiB files, serial KIO::file_copy (master) vs the batch command:

storage / cache	serial	batch	speedup
tmpfs, warm	~4k to 6k files/s	~60k files/s	~10x to 15x
SSD, warm	~3.3k files/s	~24k files/s	~7x
SSD, cold	~2.6k files/s	~12k files/s	~5x