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/types.rs

@@ -341,8 +341,10 @@ pub struct RoomUserStats {
     pub average_speed: u32,
     pub shared_files: u32,
     pub shared_folders: u32,
-    /// Upload slots the member has free; zero means they are busy.
-    pub slots_free: u32,
+    /// True when the member's upload slots are all occupied, so a download
+    /// from them will queue. The wire field is `slotsfull`, a boolean — not a
+    /// count, and not a count of *free* slots.
+    pub slots_full: bool,
     /// Two-letter country code, when the server reports one.
     pub country: Option<String>,
 }