fix: slotsfull is occupancy, not free slots, and cover it end to end

Running the e2e suite against a real soulfind showed the per-member slots
field was implemented backwards. The wire field is `slotsfull` -- a boolean
that is true when the member's upload slots are all occupied -- not a count
of free ones. `slots_free: u32` named it, documented it and printed it as
the opposite of what the server means, so a caller reading it would pick the
busiest peer to download from.

- types: `slots_full: bool`, with the wire name in the doc comment
- the DTO, the `room users` output and SKILL.md follow
- a new e2e test joins a room with a member sharing a known three files and
  asserts the counts, the roster alignment and the slot flag against
  soulfind, so the parsing is checked against a real server rather than only
  against frames this repo wrote itself

soulseek-rs/src/SKILL.md

@@ -141,10 +141,10 @@ server did not answer — those are different facts, so do not treat null as 0.
 
 **`room list`** — `room`, `users`.
 **`room users <room>`** — `room`, `user`, `status`, `average_speed`,
-`shared_files`, `shared_folders`, `slots_free`, `country`. Everything but
+`shared_files`, `shared_folders`, `slots_full`, `country`. Everything but
 `room` and `user` is `null` when the server sent no statistics for that
-member — `null` means unknown, not zero. `slots_free` of 0 means the peer has
-no free upload slot right now, so a download from them will queue.
+member — `null` means unknown, not zero. `slots_full` is true when the peer's
+upload slots are all occupied, so a download from them will queue.
 **`room listen <room>`** — streams `type`, `room`, `user`, `message`.
 **`room say <room> <message>`** — prints nothing; exit 0 is the confirmation.