internal/storage had no tests. This adds table-driven coverage for
ParseStorageURL (each scheme, query parameters, missing components, an
unknown scheme, and the fields that decide which backend is built) and a
backend-agnostic Storer conformance suite run against the file:// backend
over a temp directory: put, get, stat, list with prefix filtering,
overwrite, delete, delete-of-missing, and not-found on Get and Stat.
Tests only; no production code changed and no defect surfaced. New files
(url_parse_test.go, file_backend_test.go) avoid colliding with the
s3_test.go and file-backend work in flight on other branches. The
conformance helper takes a constructor, so s3:// and rclone:// can adopt
it once their harness is wired.
Model: opus-4-8
