feat(core,fuse,cli): reuse cached content and take several sources in moves

- A mirror download first copies the file out of the content cache when the
  same revision is cached whole or block by block, so moving a file out of an
  on-demand folder into a mirror costs no traffic for what was already read.
- `pdfs move a b c DEST` moves every source, reports each failure and exits
  non-zero if any failed.
- Mirror-to-mirror moves rename with RENAME_NOREPLACE, so a name that
  appeared at the destination after the check is never overwritten.
- The local copy a move out of a mirror drops is checked again just before
  it is deleted; anything written into it meanwhile keeps the copy and is
  uploaded as new by the next pass.

docs/USER_GUIDE.md

@@ -106,8 +106,13 @@ Drive, so it takes seconds whatever the size. For example:
 
 ```sh
 pdfs move ~/Documents/Scans ~/ProtonDrive/Archive
+pdfs move ~/Documents/a.pdf ~/Documents/b.pdf ~/ProtonDrive/Archive
 ```
 
+The last path is the destination folder; everything before it moves there.
+Files already opened in an online-only folder are not downloaded again when
+they move into a mirrored one.
+
 A move out of a synced folder waits until its files are on Proton Drive. A file that is still
 uploading, has changes that are not uploaded, or is excluded by `.pdfsignore` stops the move,
 and the message names it.