Add Cloudflare R2 file library storage backend

Implement R2FileStore, a FileStore backed by Cloudflare R2 via its
S3-compatible API, selectable with --file-store r2 and configured through
R2_* environment variables.

The store follows the MemFileStore model: a flat keyspace where directories
are derived from key prefixes, with zero-byte marker objects so empty folders
persist, and errors.ErrUnsupported for symlink/alias operations. Because R2
has no append, in-progress .incomplete uploads are routed to a local staging
directory (real O_APPEND and size-based resume) and promoted to a finished R2
object on the terminal upload-commit Rename; all other paths live in R2.

A narrow s3API seam plus an s3Uploader interface make the backend unit-testable
against an in-memory fake without a network. Adds a user setup guide at
docs/r2-file-store.md.

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