Inventories: data for slots 6-7, JSON in save files

Weapon sets and combat-art slots 6-7 live in a shadow inventory per
inventory; the slot methods run the original code on it for slots >= 5,
so item references, icon resources and selection events stay the
game's own. Save games (cInventoryManager::Serialize) and hero files
(saveHero / loadHero) store inventories as JSON ("SRIJ" + length);
vanilla blocks are still read, which migrates old saves. Multiplayer
transfers keep the binary format. nlohmann/json fetched via git.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

cmake/Json.cmake

@@ -0,0 +1,9 @@
+# nlohmann/json (MIT) — JSON for the save-game formats. Header-only.
+include(FetchContent)
+FetchContent_Declare(json
+    GIT_REPOSITORY https://github.com/nlohmann/json.git
+    GIT_TAG        v3.11.3
+    GIT_SHALLOW    TRUE
+)
+set(JSON_BuildTests OFF CACHE INTERNAL "")
+FetchContent_MakeAvailable(json)