Consider specifying "origin private file system"-ness on each "file system locator"

[a-sully]

Context: #96 (comment)
Related to #92

The cleanest way to do this would be to give each locator an optional Storage Bucket (see WICG/storage-buckets), though that API is not yet standardized

[annevk]

Can we check that the first item in path is the empty string as per #96 (comment)? I realize that is a tad magical so we could define some kind of predicate for it that has a note alongside explaining the situation.