packaging: add macOS dmg build script

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packaging/macos/build-dmg.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+BUILD_DIR="${1:-$ROOT_DIR/build}"
+DIST_DIR="${2:-$ROOT_DIR/dist/macos}"
+STAGE_DIR="$DIST_DIR/dmg-staging"
+INSTALL_PREFIX="$DIST_DIR/install"
+APP_BUNDLE="OrbitHub.app"
+
+if [[ ! -f "$BUILD_DIR/CMakeCache.txt" ]]; then
+  echo "Build directory not configured: $BUILD_DIR" >&2
+  echo "Run: cmake -S \"$ROOT_DIR\" -B \"$BUILD_DIR\" -G Ninja" >&2
+  exit 1
+fi
+
+VERSION="$(sed -n 's/^CMAKE_PROJECT_VERSION:STATIC=//p' "$BUILD_DIR/CMakeCache.txt" | head -n1)"
+if [[ -z "$VERSION" ]]; then
+  echo "Unable to determine project version from $BUILD_DIR/CMakeCache.txt" >&2
+  exit 1
+fi
+
+MACDEPLOYQT="$(brew --prefix qt@6)/bin/macdeployqt"
+if [[ ! -x "$MACDEPLOYQT" ]]; then
+  echo "macdeployqt not found at $MACDEPLOYQT" >&2
+  echo "Install it: brew install qt@6" >&2
+  exit 1
+fi
+
+mkdir -p "$DIST_DIR"
+rm -rf "$STAGE_DIR" "$INSTALL_PREFIX"
+mkdir -p "$STAGE_DIR"
+
+cmake --build "$BUILD_DIR" -j
+cmake --install "$BUILD_DIR" --prefix "$INSTALL_PREFIX"
+
+if [[ ! -d "$INSTALL_PREFIX/$APP_BUNDLE" ]]; then
+  echo "Expected app bundle not found: $INSTALL_PREFIX/$APP_BUNDLE" >&2
+  exit 1
+fi
+
+"$MACDEPLOYQT" "$INSTALL_PREFIX/$APP_BUNDLE"
+
+cp -R "$INSTALL_PREFIX/$APP_BUNDLE" "$STAGE_DIR/$APP_BUNDLE"
+ln -s /Applications "$STAGE_DIR/Applications"
+
+OUTPUT_DMG="$DIST_DIR/OrbitHub-${VERSION}.dmg"
+rm -f "$OUTPUT_DMG"
+hdiutil create -volname "OrbitHub" -srcfolder "$STAGE_DIR" -ov -format UDZO "$OUTPUT_DMG"
+
+echo "Created $OUTPUT_DMG"