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/server/mod.rs

@@ -614,7 +614,7 @@ mod tests {
             .expect("member stats");
         assert_eq!(stats["members"][0]["username"], "alice");
         assert_eq!(stats["members"][0]["status"], "online");
-        assert_eq!(stats["members"][0]["slots_free"], 1);
+        assert_eq!(stats["members"][0]["slots_full"], true);
         assert_eq!(stats["members"][0]["country"], "NL");
     }
 
@@ -819,7 +819,7 @@ mod tests {
                 average_speed: 1024,
                 shared_files: 20,
                 shared_folders: 3,
-                slots_free: 1,
+                slots_full: true,
                 country: Some("NL".to_string()),
             }]
         }