feat(output): add no-replacement output mode

Add --output-dir and [processing].output_dir so encoded files can be
written to an existing output directory without replacing, moving, or
deleting originals.

Preserve input paths below the output directory, avoid overwriting
existing outputs with numeric suffixes, and keep --backup-dir compatible
with --output-dir.

Update finalization, backup, and size-report handling so replacement
mode and output-dir mode share the same output path flow.

Document the new mode in the README and man page, add bash completion
for the new option, and expand the size report schema with input and
output paths.

Closes #7.

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