Show a mod's cosmetics as Collector, the game's sixth rarity

A cosmetic a mod installs is in no retail catalogue, so it carries no
rarity and its card shows none. Collector is the game's own sixth tier,
already styled and already worn by 395 retail items: it reads as
something apart without claiming to be Legendary, and invents nothing.

- A rarity_id from the content cache or the profile still wins, so
  letting mod authors declare one later needs no change here.
- Guarded on catalogs.available: with no pack installed
  reserved_cosmetic() answers false for everything, and every retail item
  would have turned Collector.
- Build-kit objects share this message and are not cosmetics, so they are
  left alone.

Extension/Customization/local_cosmetic_catalog.cpp

@@ -89,7 +89,17 @@ std::string cosmetic_ownable_message(const std::string& key, const CosmeticItemI
     cosmetic_wire_number(body, 10, owned ? 1 : 0);
     // The OwnableData decoder maps tag 0xba (field 23) to rarityId +160.
     // The ID references a separate server rarity definition; never infer one from an asset key.
-    if (metadata.contains("rarity_id")) cosmetic_wire_string(body, 23, metadata.at("rarity_id").get<std::string>());
+    if (metadata.contains("rarity_id")) {
+        cosmetic_wire_string(body, 23, metadata.at("rarity_id").get<std::string>());
+    } else if (!info.build_kit && content_cache::catalogs().available && !reserved_cosmetic(key)) {
+        // A cosmetic a mod installed is in no retail catalogue, so it has no
+        // rarity of its own and its card would show none. Collector is the
+        // game's own sixth tier, already styled and already used by 395 retail
+        // items: it reads as something apart without claiming to be Legendary.
+        // A rarity_id the mod or the profile gives still wins, and with no
+        // catalogue installed nothing is judged modded.
+        cosmetic_wire_string(body, 23, "collector");
+    }
     std::string framed;
     cosmetic_varint(framed, body.size()); framed += body;
     return framed;