fix(base): virtual folder entries report a 1970 upload timestamp #253

Problem

b2_list_file_names and b2_list_file_versions with a delimiter return virtual folder entries (action: "folder"). The B2 docs say their uploadTimestamp is 0. blazer converts it with millitime(0) (base/base.go), so the listing's base.File and FileInfo for a folder carry an upload time of 1970-01-01T00:00:00Z instead of "no time". This is the same pattern as #244 (key expirations).

Impact

Low. Object.Attrs() doesn't return the listing record's cached attributes for a virtual folder: with an empty file ID, it resolves the name with a download request first (b2/b2.go ~:625, ~:810). Code that reads a folder's timestamp from listing data would still see 1970 rather than a zero time.Time.

Expected

A folder entry's upload timestamp maps to the zero time.Time, so IsZero() identifies it, matching the #244 fix for keys.

Acceptance criteria

• A listing with a delimiter that returns action: "folder" with uploadTimestamp: 0 yields a zero timestamp for that entry, tested with a literal B2-shaped fixture.
• Regular file entries keep their exact millisecond timestamps (same test).
• millitime is not changed globally: other callers (upload, finish, copy, hide, get-file-info, unfinished large files) have no documented zero or null meaning.

Links

• Found while auditing millitime callers for fix(base): Key.Expires() returns 1970-01-01 for keys with no expiration #244 (PR fix(base): report a zero expiration for keys that never expire #250).
• Docs: b2_list_file_names, b2_list_file_versions