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-lib/src/client/tests.rs

@@ -632,7 +632,7 @@ fn room_member_stats_are_returned_sorted_and_scoped_to_their_room() {
         average_speed: 1,
         shared_files: 2,
         shared_folders: 3,
-        slots_free: 1,
+        slots_full: true,
         country: None,
     };
 
@@ -660,7 +660,7 @@ fn rejoining_replaces_the_previous_member_stats() {
         average_speed: 1,
         shared_files: 2,
         shared_folders: 3,
-        slots_free: 0,
+        slots_full: false,
         country: None,
     };