build: macOS universal binary (arm64+x86_64) with deployment target 11.0

- Set CMAKE_OSX_DEPLOYMENT_TARGET and CMAKE_OSX_ARCHITECTURES before
  project() so they propagate to all FetchContent dependencies (SDL3, etc.)
- build.sh: native mac release builds universal binary, detects and
  rebuilds single-arch libsodium, verifies with lipo, exports
  MACOSX_DEPLOYMENT_TARGET; dev build uses correct build/mac directory
- fetch-libsodium.sh: build arm64 and x86_64 separately then merge with
  lipo on native macOS; fix sha256sum unavailable on macOS (use shasum)

CMakeLists.txt

@@ -3,6 +3,17 @@
 # Released under the GPLv3
 
 cmake_minimum_required(VERSION 3.20)
+
+# macOS: set deployment target and universal architectures BEFORE project()
+# so they propagate to all targets, including FetchContent dependencies (SDL3, etc.)
+if(APPLE)
+    set(CMAKE_OSX_DEPLOYMENT_TARGET "11.0" CACHE STRING "Minimum macOS version" FORCE)
+    # Build universal binary (Apple Silicon + Intel) unless the user explicitly set architectures
+    if(NOT DEFINED CMAKE_OSX_ARCHITECTURES OR CMAKE_OSX_ARCHITECTURES STREQUAL "")
+        set(CMAKE_OSX_ARCHITECTURES "arm64;x86_64" CACHE STRING "macOS architectures" FORCE)
+    endif()
+endif()
+
 project(ObsidianDragon 
     VERSION 1.1.1
     LANGUAGES C CXX