Allow input of a file specifying filepaths to hash as input; produce dry-run output

If a user has a list of filepaths that they need checksums for rather than simply a root directory, they should be able to pass a CSV with these instead of a root directory to walk.

In this case, hash exactly all of the files specified by this input (no more or less).

No files, tests, or entry points are named. Start by locating the CLI path that currently accepts a root directory, then inspect its checksum traversal and output handling; the CSV input must hash exactly its listed files, but the issue does not define the CSV format or what dry-run output should contain.

Written by the indexing model from the issue text.