add build, DMG creation scripts, CI/CD workflows

- build-bundle.sh: regenerates Info.plist with KLInfo entries for all 4
  layout versions, sets CalVer bundle version, validates plists
- create-dmg.sh: packages bundle into a DMG with drag-and-drop symlink
  to /Library/Keyboard Layouts/
- release.yml: GitHub Actions workflow that validates, builds, creates DMG,
  publishes GitHub Release on CalVer tag push
- validate.yml: CI validation on push/PR for layout and script changes
- Info.plist now declares all 4 layouts (was only v2.0)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/validate.yml

@@ -0,0 +1,27 @@
+name: Validate Layouts
+
+on:
+  push:
+    branches: [master]
+    paths:
+      - 'EurKey-macOS.bundle/**'
+      - 'scripts/**'
+      - 'spec/**'
+  pull_request:
+    paths:
+      - 'EurKey-macOS.bundle/**'
+      - 'scripts/**'
+      - 'spec/**'
+
+jobs:
+  validate:
+    runs-on: macos-latest
+    steps:
+      - name: checkout
+        uses: actions/checkout@v4
+
+      - name: validate layouts
+        run: python3 scripts/validate_layouts.py
+
+      - name: build bundle
+        run: bash scripts/build-bundle.sh