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/output.rs

@@ -384,7 +384,7 @@ pub struct RoomMemberRecord {
     pub average_speed: Option<u32>,
     pub shared_files: Option<u32>,
     pub shared_folders: Option<u32>,
-    pub slots_free: Option<u32>,
+    pub slots_full: Option<bool>,
     pub country: Option<String>,
 }