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/commands/social.rs

@@ -164,7 +164,7 @@ pub fn room_users(ctx: &Ctx, room: &str, timeout: Duration) -> CliResult {
             average_speed: stat.map(|s| s.average_speed),
             shared_files: stat.map(|s| s.shared_files),
             shared_folders: stat.map(|s| s.shared_folders),
-            slots_free: stat.map(|s| s.slots_free),
+            slots_full: stat.map(|s| s.slots_full),
             country: stat.and_then(|s| s.country.clone()),
         });
     }