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/daemon/proto/dto.rs

@@ -468,7 +468,8 @@ pub struct RoomUserStatsDto {
     pub average_speed: u32,
     pub shared_files: u32,
     pub shared_folders: u32,
-    pub slots_free: u32,
+    /// True when the member's upload slots are all occupied.
+    pub slots_full: bool,
     pub country: Option<String>,
 }
 
@@ -480,7 +481,7 @@ impl From<&RoomUserStats> for RoomUserStatsDto {
             average_speed: stats.average_speed,
             shared_files: stats.shared_files,
             shared_folders: stats.shared_folders,
-            slots_free: stats.slots_free,
+            slots_full: stats.slots_full,
             country: stats.country.clone(),
         }
     }
@@ -494,7 +495,7 @@ impl From<RoomUserStatsDto> for RoomUserStats {
             average_speed: dto.average_speed,
             shared_files: dto.shared_files,
             shared_folders: dto.shared_folders,
-            slots_free: dto.slots_free,
+            slots_full: dto.slots_full,
             country: dto.country,
         }
     }