Add cancellable File::file_sync operations.

[samuel-williams-shopify]

Generic File consumers can now explicitly synchronize completed writes through file_sync(file, mode, cancellation).await. SyncMode::All requests contents and metadata; SyncMode::Data permits omitting metadata unrelated to retrieving the contents.

Linux io_uring uses native Fsync requests and checks opcode support during initialization. Buffer-free synchronization requests share submission, cancellation, and terminal-completion tracking with transfers. Readiness and Tokio use sync_all/sync_data on their existing file-operation pools, while the in-memory implementation completes immediately without modeling crash durability.

The operation is lazy, reports OS completion and errors, leaves the cursor unchanged, and retains the file through backend access even if its waiter disappears. Blocking workers check cancellation before execution; started calls preserve their result. Ring cancellation is best effort and successful completion wins a concurrent cancellation request. Callers must await preceding writes; synchronization does not order concurrent work, flush application buffers, or synchronize a parent directory entry.

This adds a required method to File, so external implementations must supply it. Update the design, public documentation, and release notes alongside shared backend contracts and cancellation/resource-lifetime coverage.