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)

build.sh

@@ -197,7 +197,14 @@ bundle_linux_daemon() {
 # ═══════════════════════════════════════════════════════════════════════════════
 build_dev() {
     header "Dev Build ($(uname -s) / $BUILD_TYPE)"
-    local bd="$SCRIPT_DIR/build/linux"
+
+    # Use platform-appropriate build directory
+    if [[ "$(uname -s)" == "Darwin" ]]; then
+        local bd="$SCRIPT_DIR/build/mac"
+        export MACOSX_DEPLOYMENT_TARGET="11.0"
+    else
+        local bd="$SCRIPT_DIR/build/linux"
+    fi
 
     if $CLEAN; then
         info "Cleaning $bd ..."; rm -rf "$bd"
@@ -732,7 +739,9 @@ build_release_mac() {
         fi
         info "macOS cross-compiler: $OSXCROSS_CXX (arch: $MAC_ARCH)"
     else
-        MAC_ARCH=$(uname -m)
+        # Native macOS: build universal binary (arm64 + x86_64)
+        MAC_ARCH="universal"
+        export MACOSX_DEPLOYMENT_TARGET="11.0"
     fi
 
     header "Release: macOS ($MAC_ARCH$(${IS_CROSS} && echo ' — cross-compile'))"
@@ -811,12 +820,31 @@ TOOLCHAIN
             -DCMAKE_OSX_DEPLOYMENT_TARGET=11.0 \
             ${COMPILER_RT:+-DOSXCROSS_COMPILER_RT="$COMPILER_RT"}
     else
-        info "Configuring (native) ..."
+        # Build libsodium as universal if needed
+        local need_sodium=false
+        if [[ ! -f "$SCRIPT_DIR/libs/libsodium/lib/libsodium.a" ]] && \
+           [[ ! -f "$SCRIPT_DIR/libs/libsodium-mac/lib/libsodium.a" ]]; then
+            need_sodium=true
+        elif [[ -f "$SCRIPT_DIR/libs/libsodium/lib/libsodium.a" ]]; then
+            # Rebuild if existing lib is not universal (single-arch won't link)
+            if ! lipo -info "$SCRIPT_DIR/libs/libsodium/lib/libsodium.a" 2>/dev/null | grep -q "arm64.*x86_64\|x86_64.*arm64"; then
+                info "Existing libsodium is not universal — rebuilding ..."
+                rm -rf "$SCRIPT_DIR/libs/libsodium"
+                need_sodium=true
+            fi
+        fi
+        if $need_sodium; then
+            info "Building libsodium (universal) ..."
+            "$SCRIPT_DIR/scripts/fetch-libsodium.sh"
+        fi
+
+        info "Configuring (native universal arm64+x86_64) ..."
         cmake "$SCRIPT_DIR" \
             -DCMAKE_BUILD_TYPE=Release \
             -DCMAKE_CXX_FLAGS_RELEASE="-O3 -DNDEBUG" \
             -DDRAGONX_USE_SYSTEM_SDL3=OFF \
-            -DCMAKE_OSX_DEPLOYMENT_TARGET=11.0
+            -DCMAKE_OSX_DEPLOYMENT_TARGET=11.0 \
+            -DCMAKE_OSX_ARCHITECTURES="arm64;x86_64"
     fi
 
     info "Building with $JOBS jobs ..."
@@ -836,6 +864,11 @@ TOOLCHAIN
     else
         info "Stripping ..."
         strip bin/ObsidianDragon
+        # Verify universal binary
+        if command -v lipo &>/dev/null; then
+            info "Architecture info:"
+            lipo -info bin/ObsidianDragon
+        fi
     fi
     info "Binary: $(du -h bin/ObsidianDragon | cut -f1)"