internal/storage had no tests. This adds table-driven ParseStorageURL
coverage and a shared Storer conformance suite, defined once in
conformance_test.go and run against every backend that can run
in-process: file:// over a temp dir and s3:// over the same in-memory
S3 harness (gofakes3 + s3mem) that internal/s3 and the not-found test
use, so a new backend inherits the contract by passing its constructor.
The rclone backend gets construction and argument-shaping tests via
rclone's in-process ":local" backend, plus the missing-remote error
path; a comment records that its data-plane operations need a configured
remote and so cannot run in a unit test.
Tests only; no production code changed and no defect surfaced.
Model: opus-4-8
sneak