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/docs/openrpc.json

@@ -772,10 +772,9 @@
             "minimum": 0,
             "type": "integer"
           },
-          "slots_free": {
-            "format": "uint32",
-            "minimum": 0,
-            "type": "integer"
+          "slots_full": {
+            "description": "True when the member's upload slots are all occupied.",
+            "type": "boolean"
           },
           "status": {
             "description": "`online`, `away`, or `offline`.",
@@ -791,7 +790,7 @@
           "average_speed",
           "shared_files",
           "shared_folders",
-          "slots_free"
+          "slots_full"
         ],
         "type": "object"
       },